If you can’t add or delete plugins in WordPress then you have probably messed up the user capabilities for your administrator roles. I did this when I moved a WordPress multi-user account to a standalone WordPress install. To fix this you are going to have to edit your WordPress database. User capabilities are stored in the database in the wp_options table as a serialized array. Understanding how the array is serialized is the key to fixing this issue. Here is how to go about it.
Editing Serialized Data
If you are going to do this then you need to backup your database. Editing as serialized array is not for the faint-hearted. Remember if you are adding or removing keys you need to update the length of the array otherwise it will break so use this information with caution.