I'm not sure how to validate the form and the error you're getting, but I don't believe he's eligible for an OWP through a relationship to you outside of a PR sponsorship application. And he has to be in status for that, which is what you're trying to do.
If he only got a two month extension when you applied, I'd assume a restoration application is likely to fail without evidence of funds and ties to Spain, and a valid reason to stay. Qualifying for common law sometimes works, if you pay the spousal sponsorship processing fee and include that as evidence of reasons to restore or extend stay.
However, there are other issues. That approach requires you not yet be in a common law relationship, which you're not yet. However, you registered a common law relationship. And if you've just had a landing interview but "registered" a common law relationship before your landing interview, that's one big problem since you would have had to update your PR application to reflect a common law partner, but that wouldn't have been possible since you weren't actually in a relationship. The registration in Manitoba may well cause you many many issues, even if you find a way to cancel it as it was in error - assume that this will come up in sponsorship reviews.
You can't sponsor him as a spouse unless you're married or are in a valid common law relationship. He can't get an open work permit unless he's being sponsored by you OR he qualifies under another program that is not based on his relationship to you.
If you declared his status as common law with you in any IRCC applications but cannot prove one year of cohabitation and/or supplied a Manitoba common law registration, you may have misrepresented.
You may wish to find a lawyer.