Verify the loader tag
Choose a Minecraft version supported by your pack, then check that the selected Placebo file explicitly lists NeoForge. A familiar mod name or a higher library version is not enough to establish compatibility. Use the loader version requested by the complete pack.
Add the jar to the profile
Open the NeoForge instance folder from your launcher. With the game closed, put the normal Placebo jar in mods and include every other required dependency. Keep downloaded installers and sources jars outside that directory because they are not the runtime mod file you need.
Check before joining a server
Launch the pack locally first and confirm it reaches the menu without dependency errors. If you play multiplayer, follow the server's supplied pack or version list. Updating the client library independently can break a combination the server administrator has already tested.