Google sign up says my phone number can't be used for verification

if the message is specifically that the number has been used too many times, there's nothing to fix on your end, google caps how many accounts one number can verify. you'll need a different number, or try the sign up again later from a phone where it may let you skip the number step.

usually means the number is already tied to too many google accounts, or it's a voip/virtual number, google rejects most of those. it can also happen if you've tried a few times in a row and got rate-limited. wait a day and try again, ideally signing up on a phone rather than a computer. creating the account from the setup screen on an android phone sometimes doesn't ask for a number at all.