How can I fix this error: Invalid operands 'Nil' and 'int' in operator '+'.?

This is telling you that score, which is basically being interpreted as nothing (or nil, or no assigned memory address), and 1, which is being interpreted as an int (or integer) can't use the operand +. In other words empty memory address and an integer can't be added together. In other words Godot doesn't know what score is. Either it's not defined or it's out of scope, or perhaps you forgot to add it as a parameter to your _on_ScoreTimer_timeout() function ... though that last one seems unlikely.