It's already been a while but I hope it still helps you or someone else: If you've written the instance address in a string, try writing it without the quotation marks. So instead of CoinFlip public coinflip = CoinFlip("0x3bd8E7eA5E3E3Fa5e70Ef89042b4b57E43F52b91");, just use CoinFlip public coinflip = CoinFlip(0x3bd8E7eA5E3E3Fa5e70Ef89042b4b57E43F52b91);