The selected serial port does not exist

Hi there,
sorry, new to this.
I am running MacOSCatalina 10.15.7 on a MacBook pro.
The Arduino is a clone (which I didn't realise when I bought it!)
When I don't have the Arduino connected (straight to computer) there is no serial port (well, there are some but not an Arduino one). When I connect the Arduino, the power LED comes on, the L LED flashes. And in Tools>Port I get /dev/cu.usbserial-1420

I've tried restarting the computer, resetting the uno and a different USB port.

The verbose error is below. Any help much appreciated.

Arduino: 1.8.15 (Mac OS X), Board: "Arduino Uno"

Sketch uses 932 bytes (2%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.
/Applications/Arduino.app/Contents/Java/hardware/tools/avr/bin/avrdude -C/Applications/Arduino.app/Contents/Java/hardware/tools/avr/etc/avrdude.conf -v -patmega328p -carduino -P/dev/cu.usbserial-1420 -b115200 -D -Uflash:w:/var/folders/_q/90gd3vnj01x25rmt59jb58dh0000gp/T/arduino_build_920335/LEA4_Blink.ino.hex:i 

avrdude: Version 6.3-20190619
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2014 Joerg Wunsch

         System wide configuration file is "/Applications/Arduino.app/Contents/Java/hardware/tools/avr/etc/avrdude.conf"
         User configuration file is "/Users/michaelmoneypenny/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping

         Using Port                    : /dev/cu.usbserial-1420
         Using Programmer              : arduino
         Overriding Baud Rate          : 115200
         AVR Part                      : ATmega328P
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :

                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom        65    20     4    0 no       1024    4      0  3600  3600 0xff 0xff
           flash         65     6   128    0 yes     32768  128    256  4500  4500 0xff 0xff
           lfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           lock           0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00

         Programmer Type : Arduino
         Description     : Arduino
         Hardware Version: 3
         Firmware Version: 4.4
         Vtarget         : 0.3 V
         Varef           : 0.3 V
         Oscillator      : 28.800 kHz
         SCK period      : 3.3 us

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.00s

avrdude: Device signature = 0x1e950f (probably m328p)
avrdude: reading input file "/var/folders/_q/90gd3vnj01x25rmt59jb58dh0000gp/T/arduino_build_920335/LEA4_Blink.ino.hex"
avrdude: writing flash (932 bytes):

Writing | ################################################## | 100% 0.16s

avrdude: 932 bytes of flash written
avrdude: verifying flash memory against /var/folders/_q/90gd3vnj01x25rmt59jb58dh0000gp/T/arduino_build_920335/LEA4_Blink.ino.hex:
avrdude: load data flash data from input file /var/folders/_q/90gd3vnj01x25rmt59jb58dh0000gp/T/arduino_build_920335/LEA4_Blink.ino.hex:
avrdude: input file /var/folders/_q/90gd3vnj01x25rmt59jb58dh0000gp/T/arduino_build_920335/LEA4_Blink.ino.hex contains 932 bytes
avrdude: reading on-chip flash data:

Reading | ################################################## | 100% 0.15s

avrdude: verifying ...
avrdude: verification error, first mismatch at byte 0x0000
         0x62 != 0x0c
avrdude: verification error; content mismatch

avrdude done.  Thank you.

the selected serial port 
 does not exist or your board is not connected


This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

Answered my own question.

Needed a new boot loader, so got an original arduino uno. Then followed the steps here:
https://support.arduino.cc/hc/en-us/articles/360012048080-How-to-burn-the-bootloader-between-two-Arduino-UNO

Have to remember to tell the programmer to “Arduino as ISP”. Worked beautifully.

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