When I clicked "Run Start" in eclipse, the program asked me to select Main class, I tried to select:
https://imgur.com/a/q5ioiwo Run configurations screenshot
I will leave the console output under each item

1.LaunchTesting net.minecraftforge.userdev

2021-08-11 16:36:22,592 main WARN Advanced terminal features are not available in this environment
Exception in thread "main" java.lang.IllegalArgumentException: Environment variable target must be set.
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:55)
Picked up _JAVA_OPTIONS: -Xmx3072M

2. Main net.minecraft.client.main

2021-08-11 16:33:47,281 main WARN Advanced terminal features are not available in this environment
Exception in thread "main" joptsimple.MissingRequiredOptionsException: Missing required option(s) [accessToken, version]
    at joptsimple.OptionParser.ensureRequiredOptions(OptionParser.java:426)
    at joptsimple.OptionParser.parse(OptionParser.java:400)
    at net.minecraft.client.main.Main.main(Main.java:77)
Picked up _JAVA_OPTIONS: -Xmx3072M

3. Start mcp.client

2021-08-11 16:35:47,137 main WARN Advanced terminal features are not available in this environment
Exception in thread "main" java.lang.NullPointerException
    at net.minecraftforge.fml.loading.BackgroundWaiter.runAndTick(BackgroundWaiter.java:33)
    at net.minecraft.client.main.Main.main(Main.java:123)
    at mcp.client.Start.main(Start.java:17)

BUT
  if I do not use gradle, everything started fine for me, but I decided to use it, because I, I think, did not load assets.

@diesieben07

2 hours ago, diesieben07 said:

What?

Follow the documentation: https://mcforge.readthedocs.io/en/latest/gettingstarted/. Do not skip any steps.

 

Thank you! I reinstalled all files except src and everything worked for me as it should