Here are several possibilities why you can not delete yourself.
In order for the app not to be without a "helmsman", there must always be at least one member with the role of "coach" in the team. That means, if I am the only coach in the team, I have to transfer the coach rights to someone else. You can read how to do that here.
It can also be that you are the holder of the premium membership. This means that you have to transfer the ownership to another person first. Here you have to go under "Settings" to the item "Teams". If you now scroll to the very bottom, you will find the point where it says that you can "irrevocably delete" yourself from the team. If you clicked here, a pop-up will open asking you to choose a person who can manage the premium in the future.
Last active member
If you are the last registered member of a team, you have to delete the team to be able to delete yourself completely. To do this, go to "Teams" under "Settings" and scroll down to "Manage membership".
Then select "Delete team name". The deletion takes three full days.