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This is indicative that the dotNet script has been used as a Custom Controls script at some point. When a script is nominated as a Custom Controls script it is locked so that no input/output parameters, files or dotNet script references can be added.
To release the lock you will need to create a new script that does not do anything and does not need to have code in it.
Go to 'New | dotNet script' and save it.
Go to 'Tools | Manage Custom Controls | Browse' and select your new script
The lock on the original script should be immediately released.