I am putting this here so that the next person that needs this information can find it:
Recently I tried to uninstall the O365 Workflow and Forms app from a site. This was not successful and kept displaying a message to "Click here to retry". Other 3rd party apps would uninstall successfully.
Turns out there was a Preservation Policy applied to the site. In order to remove the app the Preservation Policy needed to be disabled. Afterwards the Nintex App could be uninstalled then reinstalled. Following the reinstall the Preservation Policy could be re-enabled.
It would be nice if Nintex would write their app in such a way that it was compatible with Preservation policies.