Hello, the most likely cause is the name is taken

 all names on Rescreatu must take be unique, which means that a name can only be used once!

For exarmple, if someone else had the name Bob there couldn't be another pet called Bob xD

When typing a name in the name box to hatch a pet the name will turn green if its available. You could then hatch your pet!

You might have to get a bit imaginative with names.

Also check for any spaces or other characters in in the name, you can only use a-z and 1-9