Arduino 1.8.15 is the latest Arduino version.
https://www.arduino.cc/en/software
it should also default to /usr/share/arduino/lib for the install .. as this is where linux puts the core of the arduino ide.
No. Teensyduino
never supports the modified copies of Arduino from the various linux distros. We only support the original published by Arduino.cc. Get it using the link above.
the installer is still looking for arduino 1.8.12
When you run the installer, it clearly says which versions of Arduino is supports.
You can also see the installer also clearly shows which version it is. If you have a version older than 1.54-beta9, it will only support older versions of Arduino.
No, there is no Arduino 2.1 yet. Arduino hasn't even released 2.0 yet. They're still in beta testing for version 2, as you can clearly see on their website.
Maybe version 2.1 is some sort of Debian specific addition? Who knows?! We can't see your screen and at least I can not guess where you are obtaining this very wrong info about the version of Arduino's software.
it also says in the debug window Directory: /usr/share/arduino/lib checking version arduino 1.8.12 : version: "lib/version.txt" file missing this file lives in /usr/share/arduino/lib if you use the debian pkgs.
Yes, of course. Debian and other linux distros make substantial modifications to Arduino's software when they package. This is why Teensyduino never supports Linux distro packages. We only support using the original software published by Arduino.
You need to download the Arduino IDE from Arduino.cc. Pay attention to the supported versions shown in the Teensyduino installer. It will only work if you use a supported version. And again, it will never work with any version from any Linux distro. Well, not unless they keep the original structure as published by Arduino. But as far as I know, none of the Linux distro packages keep Arduino's files and structure intact. They all make significant modifications. None of that is supported.