I figured our what to do with this.
First of all, go to the link: https://runtime.fivem.net/patches/GTA_V_Patch_1_0_1103_2.exe and download it.
With this patch, open it and let it do its work. If it opens and says “Sorry, Grand Theft Auto V not installed…”, follow the steps of this video: https://www.youtube.com/watch?v=dCEfWMqAzLI
Once the patch has installed (using either method) just restart the download for FiveM and it should work. I spent about 5hrs this morning getting it to work and it did in the end. I hope this helps
I found this post by searching the forums. It really does help to have a look at the forums before reporting an error in technical support. Let me know how this method works.