I’m logged in to Trailhead as myself and working on this unit:
Scroll to ‘Preparation for the Challenges’
Click on ‘Deploy to Salesforce’ under ‘Deploy the ApexMocks open source library’
That takes me to this page
Click on ‘Login to Salesforce’
That gives me this page
“We can’t authorize you because of an OAuth error. For more information, contact your Salesforce administrator.
1800 : There was a problem in setting up your remote access”
The first time I tried it I hadn’t launched Trailhead Playground so the login url above had the domain of my Trailhead login.
I launched Trailhead Playground and the login url then had the domain of my Trailhead Playground.
I have seen other similar questions but no conclusive explanation. I have tried it on my regular computer and also on another that has never been used for Trailhead or Salesforce before.