This site is still a work in progress.
So, you managed to download the launcher and log in but you get a prompt to download FiveM even
though you already have it.
Click continue and allow it to download.
TheFamilyRP have their own version of the FiveM client. You will not be able to join server unless you accept the download.
Note: Make sure you have exceptions in your anti-virus and Windows defender. It's common for them to capture FiveM files.