Post deleted. Hardware Issue.
Please post a schematic diagram. Hand drawn is fine, with all pins and connections clearly labeled.
Did you connection the two Arduino grounds?
I suggest to get the serial communications working alone, before implementing the rest of the project. Some good suggestions for successful serial communications can be found in the Serial Input Basics tutorial.
The serial connections look OK, but how is the setup powered?
It will not work to connect 5V to Vin as shown in the diagram. It would be best to power everything from a single, regulated 5V supply, via the 5V pins.
As I mentioned, first get the serial connection working in isolation, then add the other components.
That is usually fatal, unless you have a specially designed board with a built in, good quality, high current switching voltage regulator.
Again, this 5V to Vin connection will not work:
So far, you have only muddied the waters. Much more information is required to figure out what is wrong with your setup. Please post a complete set of standard schematic diagrams for all the components and their interconnections.
But you said you had:-
Those two devices are totally different things.
While the Nano Every does have an onboard switching regulator, the Atmega328p does not, and will not take 20V.
If you have the atmega328 pins directly connected to the Nano Every pins that probably damaged one or both processors when you had two outputs connected together.
How fast does the atmega328 send data? The delays in your code may be causing the receive buffer to overflow, corrupting the data from the atmega328.
If you powered that with 24V, I'm surprised it still works.
It won't for long, though, since the on board LD1117 voltage regulator has absolute maximum input voltage of 15V, recommended maximum is 12V.
Seriously? You evidently don't pay attention.
As a matter of fact, forum members strongly disapprove of misleading Arduino beginners, for example, by suggesting that it is OK to power a Mega2560 with 24V.
If you have a custom board, then state that clearly, right at the beginning of your post. Such details are important, especially considering that you couldn't even remember which processor you are using.