Hi
I believe this warning has something to do with the Board.Txt folder, but I'm not sure what to do to clear it, any thoughts most welcome.
Andy.
You're using an old guide to supporting breadboard arduino that does not account for the final third party board spec used in recent versions on the IDE.
Why not just use MCUDude's miniCore? It supports the atmega328p and pb, as well as the smaller flash versions of those chips, with all the common clock options.... GitHub - MCUdude/MiniCore: Arduino hardware package for ATmega8, ATmega48, ATmega88, ATmega168, ATmega328 and ATmega328PB
Installable through board manager too.