@Sarah ,
Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this!
Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I shall repost your solution.
Please "Accept" the answer as this can be beneficial to other community members.
Issue:
Unable to delete domain name 'domain.com' from Default Directory
Solution:
Post trying to delete the resource once again, the issue has been resolved.
Thanks,
Kapil