I face "Due to an unexpected technical problem.." error page

I am working on Shopify OAuth with a Shopify Partners account.
I was able to set up the redirect URI, but whenever I click it, I get

**Due to an unexpected technical problem, Shopify is temporarily unavailable. Please check back in a few minutes – we’ll be up and running in no time!
**
I followed common approaches like the ones below, but they didn’t work.
Actually, the Shopify help desk only gave me this answer.
Check Shopify Status: Visit the Shopify Status page to see if there are any ongoing issues affecting Shopify services. - no problem all the time
Clear Browser Cache
Try a Different Browser or Device
Check Internet Connection

Looking forward to your reply. Thanks a lot!