Snapshot Restore 503 error

Trying to restore a snapshot archive from a multisite subsite to a single WordPress install, but the restore is throwing a 503 error.

https://static.livechatinc.com/8801096/PBVSAEKY3E/47716fde9ce88b5a78cff6e176d19e5d/Screen%20Shot%202018-07-12%20at%209.25.06%20PM.png

Please advise what should be done to correct this issue.