Your store name can't match a member's user name, even your own, so you need to open and name the store first, then change your user name.

If you already have another ID with the name you intended to use, you won't be able to create a store with this name, I'm afraid. A member account can be closed, but it takes much longer for the username to be deleted.

The best you can do is probably alter the name you want to use slightly - add in an acceptable special character, like the asterisk or underscore, or an extra word if it can fit (eg let's just say it was your other ID that's clubsquire, you should be able to open a store called club_squire, or club_squire_pipes, and then change your own ID after). 

 Edited to correct: I forgot store names can have capital letters and spaces, so it's really additional words that can distinguish between a user name and a store name. Apologies.