I understand why the tklbam-init first boot script doesn't accept the API key when it can't find a valid profile (because you need to specify a profile if one doesn't already exist). However, it should simply show the warning (that it currently does) then continue with the firstboot scripts once the user clicks "OK".
Under the current scenario, the error and info is shown to the user, but clicking "OK" drops the user back to the API screen (with the API key still noted). Then to proceed, the user must select "Skip" which is redundant as it's the only option that will work (clicking the "OK" option on the API screen, cycles the user back to the error/info).
In a perfect scenario, it might actually still be useful to still apply the API key (even if there is no profile) as then a user can still restore a backup if they wish. Although I don't think that TKLBAM itself currently supports that option. It may also cause a user issues if they forget to add a profile later.