Fix #3: Check Roblox server status. Sometimes, a Roblox game may not launch or return an error like the HTTP 503 service unavailable when you try to open it because of an on-going server trouble.

If you are asked to continue installation without flag settings in Roblox, first clean Roblox App Data and see if it helps. More fixes here.

Also, allow the Roblox Player and the ports required by Roblox through the firewall. Disable your VPN to boost your internet speed and flush the DNS cache to fix network connectivity issues.