is not a valid mod file Forge

What this actually means

Something in mods/ isn't a Forge mod: a half-downloaded jar, a Fabric mod, a resource pack, or a renamed zip.

How to fix it

Remove the named file. Check its size -- a 0-byte or truncated jar is a failed download. Also confirm it is a FORGE build; Fabric jars look identical from the outside and will never load here.

How to be sure it is this

The named file is in mods/ but has no mods.toml, so it is not a mod.

What it gets mistaken for

Libraries, resource packs and datapacks all end in .jar or get dropped in by mistake.

What to do next

Move it where it belongs, or delete it if it was a bad download. If a mod requires it as a library, the mod page says where it goes.

What it looks like in your log

Search your crash report or latest.log for any of these:

is not a valid mod file
Mod file  ...  is missing mods.toml
InvalidModFileException

The free edition covers the six most common startup failures, and this is not one of them — it is in the paid tool, which checks all 24 and names the mod jar from the stack trace. The fix above is the same one it prints, so you do not need it to solve this.