How to connect your new Developer Edition org to Trailhead?

Hi, @Kristina Tan

Open Hands-On orgs:

https://trailhead.salesforce.com/users/profiles/orgs

Here you can see a list of your Salesforce orgs and, at the top, two buttons: Create Playground and Connect Org.If either of these buttons is inactive, it's because you've exceeded the limit of available organizations. To resolve this, delete old Playgrounds to free up space.

Sincerely, 

Mykhailo Vdovychenko 

Bringing Cloud Excellence with IBVCLOUD OÜ