Missing or unsupported mandatory dependencies Minecraft server

What this actually means

One of your mods declares a hard dependency that is absent, or present at the wrong version. Forge refuses to start rather than load a mod whose requirements aren't met.

How to fix it

Install the named dependency at the version range the log asks for. Watch the version range syntax: [1.2,) means 1.2 or newer, [1.2,2.0) means 1.2 up to but NOT including 2.0. A dependency that is 'installed' but one major version off counts as missing.

What it looks like in your log

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

requires
Missing or unsupported mandatory dependencies
requires
MissingModsException

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.