Quick answer: Unity IL2CPP development build refusing the managed debugger connection? Build doesn't include the debug stubs - enable Script Debugging in Build Settings.
Built a dev player; attached VS debugger. Could not connect: no debug server.
Enable Script Debugging
Build Settings > Development Build + Script Debugging. Required for IL2CPP debugger.
Open the debugger port
Firewall may block port 56000. Test connection with telnet first; firewall the second suspect.
Verify with the build report
Build output: Script debugger enabled. Missing = checkbox didn't apply (rare; usually a build cache issue).
“IL2CPP debugger needs explicit support. Default builds exclude it.”
For console targets where dev menus are gated, ship one dev build per QA cycle. Saves the rebuild on every debug session.