Forge version: 1.12.2-14.23.5.2854
Twilight Forest version: (1.12.2-3.9.974-universal)

Steps to reproduce:

1. I downloaded the TF mod and forge
2. Put it in the mods folder in AppData/.minecraft/mods, and in the server file

What I expected to happen:
I expected that it works both in single player and on my server.

What happened instead:
It worked in single player, but on my server I could see every single item of the mod in creative, but when I took the item and placed it on the ground it appeared for half of a second and instantly disappeared.

PS. the minimap mod worked fine

Any of the following could have occurred:

1. The server is running on vanilla rather than Forge. If the /forge command shows up as an invalid command, the server is vanilla.
2. Twilight Forest is not installed on the server. If the /forge command did run but you are unable to place blocks, then that means the mod isn't on the server, only on your client.

This means you haven't installed it correctly on your server. Basic troubleshooting is needed on your end.
1.) Does the /forge command work? If not then you're running a vanilla server.
2.) Does the /tffeature command work? If not then you haven't placed the mod jar in the correct location. {server root}/mods/

Going to close this as it's a generic issue not a Twilight Forest specific issue.