If this library is associated with an active protection group, modify the protection group to associate it with another library ...

1) If this library is associated with an active protection group, modify the protection group to associate it with another library to prevent tape-related jobs from failing. 
2) If you want to use this library for your tape-related jobs, on the Libraries tab in the Management task area, select this library,  and then click Enable library in the Actions pane.