Practical advice: If you want to/vote to migrate something to another site (especially math.SE) please make sure that at least one of the tags on the question exists on the target site (math.SE). Possibly, do a retag to achieve this. If not, the migration will be rejected.
Update: The request on meta.SO got declined. I think there is not very much for MO to be done, as mainly we migrate to math.SE and the problem can reasonably be only fixed there. (To fix the issue on the target site, is also the 'official solution', see Shog9's reply on meta.SO.) Fortunately, there is already some activity on this on meta.math.SE.
I just learned that a migration is automatically rejected if none of its tags exists on the target site.
This seems like a problem to me, since while math.SE evidently has tags for group theory, number theory and so on, they are not identically named as here on MO (since we have the two letter prefixes, mainly).
And, apparently, tag-synonyms are not even taken into account (as a question then tagged gr.group-theory which has group-theory as synonym got autorejected, while that later tag exists on math.SE).
Personally, I think this automatic "sanity check" based on tags is not a good idea, and would simply drop it. This however might be complicated to push-through, so a more modest suggestion:
Include tag-synonyms in this automatic check.
In this way, it would be possible to make the most common tags compatible (actually several of them would simply become compatible based on existing sysnonyms) while not changing the actual tags on either of the two sites.
For the moment I tagged this discussion, so that opinions on the genaral matter can be voiced and other suggestion to reseolve it might be made. Depending on how things progress I might change to feature-request.
Added: Thank you for the feedback. As a result of this dicsussion I placed a feature-request to remove this entirely with the request for synonyms to be taken into acount on meta.SO