Could you post the error that you are seeing?
Did you select the "Most Recent Backup" option when doing the manual snapshot? In that case, it looks for the latest Full or Differential backup based on msdb.dbo.backupset in the backup location. Does that backup exist there?
If you want to take the snapshot using a specific full backup, you need to select the "Specific Backup" option, and provide its backup UUID when doing the manual snapshot. The backup would have to be more recent than the latest log backup that was ingested.
Do let us know if this solves the issue that you are seeing.