Due to a change GitHub made to the infrastructure we use to produce the Arduino IDE releases, 2.3.4 is the last version that has compatibility with Ubuntu 18.04:
https://forum.arduino.cc/t/arduino-ide-2-3-4-is-now-available/1329167#p-7972984-deprecation-notice-upcoming-cessation-of-support-for-linux-distros-using-glibc-228-1
Recent changes in the framework used to produce automated release of Arduino IDE resulted in the loss of compatibility of the Linux builds with older Linux distro versions that use version 2.28 of the GNU C Library Ubuntu
Future releases (including nightly and tester builds ) will fail to start on machines using these distro versions with an error like:
Error: node-loader:
Error: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /home/foo/arduino-ide/resources/app/lib/backend/native/pty.node)
at 85467 (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:2766)
at __webpack_require__ (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:6663105)
at 23571 (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:3374073)
at __webpack_require__ (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:6663105)
at 55444 (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:3369761)
at __webpack_require__ (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:6663105)
at 24290 (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:1780542)
at __webpack_require__ (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:6663105)
at 43416 (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:1770138)
at __webpack_require__ (/home/foo/arduino-ide/resources/app/lib/backend/main.js:2:6663105)
If you don't know which version of glibc your machine is using, run this from the terminal:
/lib/x86_64-linux-gnu/libc.so.6
Further technical details are available here
The recommended path forward is to update to a modern version of Linux. If you can't or won't do that, you can update to the last compatible version of Arduino IDE, 2.3.4, and then avoid updating when future releases (e.g., 2.3.5) come out. There are two methods for dealing with these update offers. I'll provide instructions for both of them. You can pick whichever one of the two is most convenient for you.
Ubuntu 18.04 is quite outdated.
If for some reason you won't or can't update your operating system, you can download Arduino IDE 2.3.4 from the links under the "Assets" section of the 2.3.4 release page on the Arduino IDE GitHub repository: