Target type of a lambda conversion must be an interface

[JacobeTerminator]

Hello! I create a only server-side mod and i have problem with my ide. 

 

ModLoadingContext.get().registerExtensionPoint(ExtensionPoint.DISPLAYTEST, () -> Pair.of(() -> FMLNetworkConstants.IGNORESERVERONLY, (a, b) -> true));

My IDE tells  "Target type of a lambda conversion must be an interface"

 

My imports here: 

import com.mojang.datafixers.util.Pair;
import net.minecraftforge.common.MinecraftForge;
import net.minecraftforge.fml.ExtensionPoint;
import net.minecraftforge.fml.ModLoadingContext;
import net.minecraftforge.fml.common.Mod;
import net.minecraftforge.fml.network.FMLNetworkConstants;

Minecraft 1.16.5

[JacobeTerminator]

Sorry, I've only been learning java for 3 months and recently started learning lambda expressions, could you help me and tell me what's wrong with this code.