The image is simple indicating to me that the secondary operations for those SLPs are disabled. This should not stop you selecting this SLP as a storage unit override.
You can only enalbe secondary operations in the SLP section of the GUI (right click on an SLP and select "activate secondary operations). Whether or not secondary operations are disabled, this should not stop the initial backup writing to the "backup" STU of the SLP (it will just stop any duplication or replication happening).
To be complete, if the first operation in the SLP is a snapshot, then the above may not be 100% correct.
If you are actually prevented from selecting one of the SLPs let us know.