When you have knowingly or unknowingly unchecked the user/user_permission from the modify permission list of opencart Users > User Group > Administrator, you will not be able provide yourself the permission to check or uncheck any of the extensions or modules in the list.
In such cases, to get back your access, you will have to temporarily disable the validation check in the code. Open file admin/controller/user/user_permission.php and find the function
protected function validateForm()
and comment the line
//$this->error['warning'] = $this->language->get('error_permission');
Here is the complete video demonstration
If you need any assistance, feel free to write to me at firstname.lastname@example.org