As a photography CC subscriber, Why can't I update from Lightroom 5.7 to 5.8?
There is no LR 5.8. The last version of LR 5 was 5.7.1: Update information for older versions of Adobe Photoshop Lightroom
If you mean you can't update from CC 2015.7 to CC 2015.8, what happens when you try?
Retrieving data ...