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Please see the official X‑Plane Support page for help.
First, check that you have not remap the Enter key to operation other than to the default ATC.
Second, it’s possible that some plug-in could be taking over the Enter key. For example, in “XSquawkBox”, I have set up the Enter key to do just that: pop open a line to enter text message.
I am not from Laminar Research; just a flight simmer from down under.
Follow the advice provided by disc but also check that your system also meets the system requirements which can be found at the following link https://www.x-plane.com/kb/x-plane-11-system-requirements/ Many flight simmers go for "better than" or "equal to" the recommended requirements.
Also check that your GPU drivers are up to date. Disregard what your operating system tells you. There has been a recent case where a flight simmer was having trouble running his system for two years. His OS kept telling him that the drivers were current but after some research downloaded the current driver and everything then ran perfect.
Also make sure you are running XP as the administrator.
This site is no longer being actively maintained.