_exists function error

Solidity Programming
#1

from solidity:
DeclarationError: Undeclared identifier.
–> CryptoR1155.sol:48:13:
|
48 | require(_exists(tokenId), “ERC1155Metadata: totalSupply query for nonexistent token”);
| ^^^^^^^

#2

It says that _exists function is not defined?

#3

yes this the error but _exists is an internal function

#4

the function that it is using it is also internal?