We have lower than usual capacity in the region your app is deployed in. I’m gonna copy @senyo’s great response to someone about this earlier this month:
One way around this is to create a volume in the same region and then do all the migration steps manually.
Otherwise, if you’re not a rush, you can retry the migration periodically. The worker may have capacity at a point in future and we’ll be able to run the migration successfully.
For what its worth, this issue is unique to the way we’ve designed volumes in that they are tied to a host, hence the issue. We’ve got ideas to make this work out of the box so in future, this shouldn’t be a problem.