Start with the right checks.
Joining Wi Fi and reaching the internet are separate things. A comparison with another device helps establish whether the Mac, the network or an outside service needs attention.
What to do next.
Check the intended Wi Fi network and try two ordinary websites.
Try another device on the same network. Note whether it has the same problem.
Save work and restart the Mac normally. Check its date and time.
If available, review Wi Fi Recommendations. Tell the administrator about VPN software or recent network changes rather than disabling them without approval.
Check the result.
Confirm access to the original website and an everyday application. A connection that works on another network is useful evidence for further diagnosis.
Stop and get help if…
Several devices are affected, the router reports an outage or the problem returns. Contact the network owner or internet provider as appropriate, with the comparison results.