A tModLoader mod did not load on your server

Read the Console from the start of the last boot. The lines that begin with [pnh-mods] say which Workshop mods downloaded and which failed, and tModLoader's own lines after them say which mods it could not load. Then type modlist to see what is running. The usual causes are a wrong id, a missing required mod, or a mod made for an older tModLoader.

The [pnh-mods] lines

A required mod is missing

Many mods need another mod or a library. On the Workshop page, a mod's row says Needs and the names when one is missing, and Add all adds them. Click Save and Restart to apply. See How to add Steam Workshop mods to your tModLoader server.

Required mods not checked yet. means the panel could not ask Steam yet. Open the page again in a few minutes.

The mod is too old

A mod last updated before the tModLoader version your server runs can download and still not load. Look on its Workshop page for an update, or remove it. After a tModLoader update the Console says Mods made for the previous release may need an update too.

The server stops while loading mods

Read the last lines of the Console and look for an error that names a mod. Then check Overview: only if it shows This server ran out of memory did the server run out, which large mods such as Calamity can cause: Your Terraria server ran out of memory. Otherwise, removing the newest mod and restarting tells you which one it was.

A mod you uploaded yourself

Its name must be in mods/enabled.json, spelled as the .tmod file name: How to upload your own mods to your tModLoader server.