Received lump sum rollover check from prior employer Pension and trying to use the Vanguard App mobile deposit with no luck. Have tried my phone, wife's phone(both Android) and an Ipad. Keep getting error messages having difficulty processing your check. Any suggestions? I know I can mail into Vanguard but would prefer not to if at all possible.

Make sure your OS and the app are updated to latest versions.

See if that helps. Apple pushed out a new iOS the last week. Your phone may not be showing it. 14.7.1 is the latest.

It may also be the dollar amount exceeds their limit.

Or it's Sunday and their system is going through a standard Maintenance.

Try again tomorrow. See what happens