After installing the macOS Big Sur update, some Mac owners encountered an error when trying to install Steam: "Steam needs to be online to update. Please confirm your network connection and try again."
Unfortunately, this is a long-running Steam issue, Mac users have been at least 10 years old, and there is no centralized fix, and it is not clear what caused the error. On Steam servers that reappear occasionally, this seems to be a problem.
Although Valve does not provide an official fix, there are many solutions that seem to work for people who encounter this problem, so if you encounter this error, the following suggestions can solve the problem.
Try the hotspot of your phone
Connecting to the hotspot of your smartphone instead of your home WiFi to download Steam seems to be one of the fastest and easiest solutions people use to successfully fix this error.
It's as simple as switching to a smartphone hotspot, downloading the software, and then switching back to a standard WiFi connection. After installing the update, Steam should open normally.
Unfortunately, this solution does require a stable hotspot connection, so if you don't have a solution available, you can use some other fixes.
Change your default DNS server
Steam’s error may be related to DNS, so some users have seen the results of changing the DNS server to Google’s public DNS address (8.8.8.8). By opening "System Preferences", selecting "Network", clicking "Advanced" and selecting the "DNS" tab, you can access the DNS settings on your Mac.
Use VPN
Before installing the update, some Steam users have been able to skip the error message by changing their IP address using a VPN.
Download and reinstall Steam again
A few reports suggest re-downloading Steam and re-attempting the installation to successfully solve this problem, but most people have to use other methods to make the installation work.