So, uhh I too have this issue, but I can fix it by switching from DX11 to DX12. But, the game crashes for me when playing DX12, and it does it basically every ten minutes or so.
Please make sure you have no mods running and try verifying your files.
If you can, try running the game in the other DX mode, for example, if you are running the game in DX12, try running it in DX11 and vice versa.
Remove any additional apps (example UDP apps)
Try completely shutting down the game.
So everything here I've noted and checked and of course the main issue with not able to join sessions gets fixed with DX12, but it's quite a pain when the game keeps crashing.
Was it your first time trying Multiplayer? It was not my first time. I've done placements plus some more ranked and couple of league races before.
Which platform you are on PC, Steam
Your relevant Steam ID/PSN ID/Gamertag so we can check further. Steam Profile
If you're on Steam, can you let us know what version of Windows you are on Windows 10 64bit ver. 1903