Make sure that in the AMP web page you are:
creating a new instance, that is of the application type “minecraft java”
once the instance has started click manage,
click configuration then proceed to server settings,
here you will select the server type, make sure you select Forge,
after the settings automatically save click download/update,
wait until the notification goes away, it is installing the latest version of forge,
after that has completed you may select your release stream, if you are running a modpack I highly recommend using “specific version”,
then select your Forge Version, make sure it matched what client you want to use to connect with,
I would then click download / update again and let it download that version of forge that you specified
after that has completed if you are running a specific modpack, make sure you got the server jar file and upload that via the file manager,
once that has been uploaded or if you just want to play vanilla forge go back to server settings and click “scan for new jars”,
once that is done you may select it from the dropdown box
if you did all that without an error you would have successfully installed a forge server
if you run into a network problem it is likely your firewall either on the server or router, if you run into critical errors it could be because you do not have the right version of java installed, try updating your system.