Environment Details
In certain situations scheduled workflows will stay queued.
Cause
Cause A
The Service Account AlteryxService.exe has insufficient permissions.
Cause B
None of the Server nodes has the option "Run unassigned jobs" ticked in System Settings > Worker > General.
Resolution
Solution A
Make sure AlteryxService.exe is running under a Service Account with the permissions listed in https://help.alteryx.com/server/current/admin/Configuration/RunasPermissions.htm.
Solution B
Make sure at least one Worker node has the option "Run unassigned jobs" ticked.
Additional Resources
KB Article: Error: Workflows Queued - 500 Server Error Key fields must be a WString AS Schedules ID
Help Doc: Required Run As User Permissions