I can access Coinbase either without any problem.
If you can't access Coinbase and always getting this error maybe the problem is not from Coinbase?
Can you try to clear the browser cache or try to access the website through incognito mode?
Or try to change your DNS to Google DNS or Cloudflare DNS and do this below.
Open the command prompt and enter these.
ipconfig /release
ipconfig /renew
ipconfig /flushdns
After that try to access Coinbase again and let see if the issue still persists.