You need to select the NeoForge server type and NeoForge version for it to start properly, since there isn’t a proper Jar.
Guide here for the full list of steps:
Forge Frequently Asked Questions
How do I run Forge using: start.sh/start.bat?
No need! AMP handles all that for you. Just select the version of Forge you’ll need, hit update, and start your server.
Where do I put custom start up arguments?
You can find this options under:
Configuration → Java and Memory → Additional java options
I installed a modpack using AMP’s FTB downloader, why did it start in Vanilla? or The update manager reported a successful update, but ./Minecraft/minecraft_server.j…