You have the default mail type set to "First Class" and first class mail is only available for packages under 13oz.... The code detects that and sets the mail type to "PARCEL" when it is oversized for first class and first class is selected. Well it turns out that PARCEL also has restrictions as identified in the error message.
I've updated the code to use "PRIORITY" when first class is selected and not available. That is the standard USPS mail type and it should be available everywhere.