Here are several possibilities why you can not delete yourself.
Coach rights
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.
Premium owner
You may also be the owner of the Premium membership. This means that you must first transfer the ownership to another person. To do this, click on your profile at the bottom right. Then go to "Edit teams", which is located next to the team settings. If you now scroll down to the bottom, you will find the point where it says that you can "irrevocably delete" yourself from the team. Once you have clicked here, a pop-up will appear asking you to select a person who can manage the Premium in the future.
Last active member
If you are the last registered member of a team, you must delete the team in order to be able to delete yourself completely. To do this, go back to the team settings under Profile and scroll down to "Manage membership".
Then select "Delete team name". The deletion takes three full days.