I cannot delete a project

Hello,
Does your profile allow you to modifiy or delete a project ?
You can check what your profile is on the user parameters menu (on the top right).
For instance, here, it is "Administrator"


Then, on the "Access to data (project dependant)" screen ( in Configuration/Access Rights), check what your profile can do with "Projects".
Here, the administrator is "manager+" meaning he can delete all elements in all projects.

Indeed, when you're allocated to a project with a profile "lower" than your default profile (in your case, from Admin to Supervisor), then you get the rights of the profile you have on this project.

Just a side note : it might be better in such a case to change your allocation on this project (for yourself only) to be an "Admin", instead of a Supervisor (and then, be able to delete the project, as you wanted to do).
If you change data access for Supervisor, please take into consideration that you now allow every Supervisor to delete their projects. Just be sure it's what you want to do.