Ive had this problem before, try using the latest server artifacts, or the latest recommended. Also delete the servers cache, and clone the default resources back into your resources folder. (Of course be sure to delete the old “stock” resources first.)
To start with I would see if the issue lies within human error. Meaning did YOU edit the Chat Script itself? No then it might be best to clear the servers Cache as mentioned by @JeffPolar You may also indeed download the Default Resources For FiveM. You could also go here to setup a server from the beginning and see if the chat works there. I had this issue for a while…
Close the server.
Delete the Servers cache (Reboot may take a little longer depending on resources)
What I stated was meant for the older builds. And however sometimes the chat resource will still fail to load even after it downloaded what it needs. I personally had this happen to one of my development servers. After just re-downloading it it worked fine.