It says my NFT's collection is not registered for auction

Only collections that have been enabled on Creta's auction contract can be listed here, and that message means yours is not one of them. It is a limit of the marketplace, not a problem with your item — the NFT is unaffected and still yours.

Creta's own collections are enabled, and so are NFTs you mint on this site. An NFT bought elsewhere, on another marketplace or from an outside project, generally is not, and cannot be sold here.

You can still transfer such an item out of your wallet, or sell it wherever its own collection trades.