The "php" plugin does not exist
It's a guess, but the php module was removed from core in D8 and is now supplied by a contrib module: PHP

You will need to install it additionally.



Edit 1: 

If you don't intend to use the module in the future, enable it only temporarily, find alternatives for the fields/text formates currently using it (if needed, the filter may be available but not actually used for php) then uninstall it again once you are done.