i2501

Based on quick googling, have you tried:

-Waiting >24 hours since the last time you attempted it? Square's system apparently has an internal C/D which it'll just keep throwing that error if you try to pursue it without waiting 24 hours since the last time you got the error as an anti-scam/etc system.
-tried using a different computer/use your phone? Apparently it can be a browser issue and switching to a different PC to do the payment/trying your phone phone can solve it.
-Turned off VPNs/Proxies? The mogstation REALLY doesn't like VPNs, as my own experience has taught me.

Try purchasing through your phone. Worked for me.