But it is at 1.15.2 also:
2021-07-06 21:20:35,919 main WARN Advanced terminal features are not available in this environment
←[32m[21:20:36] [main/INFO] [cp.mo.mo.Launcher/MODLAUNCHER]: ModLauncher running: args [--gameDir, ., --launchTarget, fmlserver, --fml.forgeVersion, 31.2.50, --fml.mcpVersion, 20200515.085601, --fml.mcVersion, 1.15.2, --fml.forgeGroup, net.minecraftforge]
←[m←[32m[21:20:36] [main/INFO] [cp.mo.mo.Launcher/MODLAUNCHER]: ModLauncher 5.1.2+70+master.2845bb9 starting: java version 16.0.1 by Oracle Corporation
←[mjava.lang.NoSuchFieldException: ucp
at java.base/java.lang.Class.getDeclaredField(Class.java:2549)
at cpw.mods.gross.Java9ClassLoaderUtil.getSystemClassPathURLs(Java9ClassLoaderUtil.java:28)
at cpw.mods.modlauncher.TransformationServicesHandler.discoverServices(TransformationServicesHandler.java:139)
at cpw.mods.modlauncher.Launcher.run(Launcher.java:74)
at cpw.mods.modlauncher.Launcher.main(Launcher.java:65)
at net.minecraftforge.server.ServerMain$Runner.runLauncher(ServerMain.java:63)
at net.minecraftforge.server.ServerMain$Runner.access$100(ServerMain.java:60)
at net.minecraftforge.server.ServerMain.main(ServerMain.java:57)
Exception in thread "main" java.lang.NullPointerException: Cannot read the array length because "urls" is null
at java.base/jdk.internal.loader.URLClassPath.<init>(URLClassPath.java:153)
at java.base/jdk.internal.loader.URLClassPath.<init>(URLClassPath.java:174)
at java.base/java.net.URLClassLoader.<init>(URLClassLoader.java:149)
at cpw.mods.modlauncher.TransformationServicesHandler$TransformerClassLoader.<init>(TransformationServicesHandler.java:159)
at cpw.mods.modlauncher.TransformationServicesHandler.discoverServices(TransformationServicesHandler.java:139)
at cpw.mods.modlauncher.Launcher.run(Launcher.java:74)
at cpw.mods.modlauncher.Launcher.main(Launcher.java:65)
at net.minecraftforge.server.ServerMain$Runner.runLauncher(ServerMain.java:63)
at net.minecraftforge.server.ServerMain$Runner.access$100(ServerMain.java:60)
at net.minecraftforge.server.ServerMain.main(ServerMain.java:57)