So how has Sony handled this issue? The manufacturer has suggested that if your Xperia Z5 won't open the Google Play Store following the update to Android 6.0, a factory reset might do the trick. And while it has solved the problem for some, there are still many Xperia Z5 users that still can't access the Play Store even following a factory reset.
Based on responses to the Sony support forum, the best way to fix the problem is to sync your Xperia Z5 to the PC on your desktop, and head to PC Companion (http://support.sonymobile.com/gb/tools/pc-companion). There, you will be able to re-install Android 6.0. This time however, you won't lose the ability to open up the Google Play Store.
Let us know if this did the trick by dropping your comment into the box below.
Thanks for the tip!