First, verify that you can see your user created in IAM identity center. I have created one user named "pachispachis". I suggest you add it to an user group for administrators.

After that, head over to permission sets (under Multi-account permissions), verify there is an administrator access permission set created. After that, head over to AWS accounts (under multi-account permissions) and SELECT the AWS account you are giving access to, and click on "Assign users or groups".

After that, it will ask you to select users and groups to give access to the account, you add the user or the group where you user is (in case you added your user to a group). You will then be asked to select the permissions sets associated to that user/group:

Go ahead and review and submit it. It might take a few minutes for your user to get access to the account (see it in the aws portal access). Verify the user has access under "Users" section (in IAM identity center).

Let me know if that works and/or if other issues ocurr.