Can't select certain groups for giveaway?

Is this because of characters e.g. Couples Games has a space in it?

Definitely not. I've just checked my groups, and found 2 that have a space in the middle, and have many giveaways made, in one of them even on day 1 already.
Quick search resulted in finding these 2 ancient discussions: one two
But they don't really give any other advice than to be patient... :/

I think I can explain. It's usually a problem with group Sync of steam API. it happens with some new groups. In about two weeks, one month at most it should be fixed by itself.

steamgifts cache need some time to work :)

I selected Follow on your curator page, but when I search Steam Groups for "Couples Games" I don't get any hits.
I suspect you have a Curator page with 50ish followers, but that's different than having a group with 50ish members.
That would explain SG never seeing it, I think.

Update because I said I would: Can now select Couples Games as a giveaway group, so if anyone is reading, that gives you an indication on time frame. Ten days from when I made my first post. Closing the thread now for posterity. Thanks to everyone!