#error The library is not compatible with AVR boards

Just found out that the library was no tfor my board

i followed your github link; it says near the top: Powersave features for SAMD boards

Although you will find that some Arduino libraries work with any board, other libraries use architecture-specific code. These libraries are written for specific microcontrollers and are not compatible with any microcontroller they weren't written for. That is the case with the Arduino_LowPower library. It simply isn't written for the AVR architecture of your Nano's ATmega328P microcontroller. It has no chance of ever working with your board. It's not a bug; it's simply the way the library was designed. If you were using a SAMD architecture board, such as one of the MKR boards or the Nano 33 IoT, then the library would work fine for you.

If you look around, you will probably find a similar library written for the AVR. You can also have a look at this AVR-specific article to learn how to do it without a library: https://www.gammon.com.au/forum/?id=11497