Hi - I went through that thread and I am having similar issues.
It is pretty compelling that the error description does not give more details on what is going wrong than “We are unable to complete your request at this time”…
Is there any way this problem can be seriously solved by Square? As it stands: it looks like subscriptions are just not working…
Do you see the declined payment on your bank statement? Also did you attempt the payment again? 
Well I believe I found the root cause of the issue: I believe the subscription functionalities require to be Square plus (does not work with free tier).
I just hope there were clearer error messages back pointing to clear reasons as of why things fail (this is common weakness with square that I notice): here we just get a “we are unable to complete the request”. So I suspect many people are struggling with subscriptions api without realizing it is a premium plan issue.
