It's odd that you are getting that particular error message, and I'll check with my team about that. However, I can tell you why you're unable to purchased a guided transfer right now:
Guided Transfer is not currently available for blogs that use custom fonts. To purchase a guided transfer, you'll need to switch your custom fonts back to your theme's default fonts.
To do that, you can go to Appearance > Customize in your dashboard. Then, click on the Fonts section and click the "x" next to each custom font. Be sure to save your changes, then go back to the Store in your dashboard.
That should make it possible for you to purchase the guided transfer for your blog. :) If you run into any further trouble with that, please let me know!