Issue
Users with list permissions, but not any site-level permissions, are unable to add items or open Nintex Forms on a SharePoint List.
Error Code
"Sorry, this site hasn't been shared with you."
Resolution
Update to Nintex Forms version 5.2.1.30 (April 2021 release)
Additional Information
This issue was identified as a defect in Nintex Forms for SharePoint 2019 and has been addressed in version 5.2.1.30.