I am trying to pass the challenge described in this link, https://trailhead.salesforce.com/en-MX/content/learn/modules/data_security/data_security_org?trail_id=force_com_dev_beginner#, but my 2 Salesforce licenses with the System Administrator profile, they are sold out, however when I check the users, there is only one with such a license, coincidentally mine. Unfortunately I can not perform the challenge because I can not find the other user with this license to disable it. If this has happened to someone and you can help me, please, I will be very grateful
Rolling up from comments with more details.
While you can use your personal Developer Edition Orgs for Trailhead Challenges, but in this scenario you can always create a new Trailhead Playground and utilize it specifically here. It's also recommended that you create a new Trailhead Playground for challenges so that you don't have to worry about quite a few things that you may not have in your personal Developer Edition Org, e.g., My Domain.
In your case, you can create a new Trailhead Playground, create two Users, both assigned to your email, so that you have credentials for both users. And then you can go ahead with the challenge to activate/deactivate the other one.