Cannot complete Trailhead Create and Configure an External Client App because Require Proof Key for Coke Exchange (PKCE) extension for Support Authorization Flows is selected by default and cannot be deselected

Hi Angel - good news: this is expected platform behavior, not a bug in your org, and it probably won't actually block the challenge. 

Why you can't deselect it: Salesforce now enables 'Require Proof Key for Code Exchange (PKCE)' by default and makes it non-editable on External Client Apps. It's part of the secure-by-default hardening (the OAuth 2.1 direction), so every org behaves this way now - a fresh Playground would be the same. So you're not doing anything wrong; the greyed-out checkbox is just the current default. 

What to do: run Check Challenge with PKCE left on anyway. The validator for this step checks things like the app's name, OAuth scopes, callback URL, and the flows you enabled - not whether PKCE is switched off. So even though the older instructions may tell you to uncheck it, the challenge usually still passes with it on. Try that before assuming you're stuck - I'd bet it goes green. 

If it genuinely fails on that step: this is a known gap where the platform changed but the module text hasn't caught up yet. The right channel is a Trailhead Help case - they're who can look at a challenge that won't validate. One quick note: this Trailblazer group is peer Trailblazers, not Salesforce Support, so posting an Org ID here won't reach Support; Trailhead Help via the Help site is your fastest route, and you don't need to share your Org ID publicly to get it looked at. 

Hope that unblocks you!