This tool help you to determine if a name is valid (1), however it does not check if a name is taken in a contract. If Metamask indicates the transaction could create an error, it's probably that the name is already taken.
In the contract, "ethereum" and "Ethereum" are considered the same name and can not be taken both at the same time.
(1) Valid characters for names are: space, numbers (0 to 9), letters a to z (upper and
lowercase), coma, space and -
All others characters will make a name invalid (including dots and # sign).
No leading or ending space, no double space.
Max length of 32 characters.