SharePoint 2019 Solutions with sandboxed code are disabled

I have a SharePoint 2019 server with a single server farm role. I have enabled the sandbox code service following these steps: enable-sandbox-solutions-on-sharepoint-2016
When I upload a sandbox solution containing a workflow the activate button is enabled. When I click activate I get the ULS error: “Solutions with sandboxed code are disabled”
And on screen “Activation of solutions with sandboxed code has been disabled”

any ideas?

When I create a new Sandbox solution in Visual Studio and upload this to my site I can activate the solution.

Are workflows no longer allowed in Sandbox solutions?