This is my first SQL Conversion and I have a problem where the users seem to be denied access to the server.

I am using VDF 19.1 with SQLExpress and I logged in on the server and installed SQL and compiled the new VDF app - everything fine. On each workstation I installed the VDF 19.1 client. When VDF is run on the workstation (by a user) it asks for a login and then errors out that the server cannot be found. Here are the 2 dialogs I get:
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I could use some direction on where to look for the solution, please.