Some people can’t have the ability to keep having discord installed. You could make them manually verify with some sort of central storage system of Fivem accounts and discord id’s. You could also make use of Login with Discord feature for logins if you implement this.
I know most of the people that I talk to don’t have the option of having the desktop application installed but they use the mobile or desktop version.
Central storage system would cost a considerable amount of money for the roughly 100,000 concurrent users of FiveM. That data would need to be obtained, logged, then fetched by the server if they chose to use Discord Identification. Alternatively, servers are not required to use Discord authentication to allow users on their server.
Identifiers are meant to be seamless. I’m not sure why a user would be able to run FiveM, but not a messaging app.
And as above, you are not meant to expect the Discord identifier to exist.