This is a known issue when using the official Play Store with microG. In my own experience, I’ve seen it usually happen the first time the Play Store is opened after a reboot. You may be opening it for the first time and find a Google login page, log in, and then find that the Play Store can’t connect to the server.
The fix is simple: First, make sure that you have enabled Device Registration and Google Cloud Messaging in microG settings. Then, close out of the Play Store completely - including removing it from the recent apps list - and open it again.
If that doesn’t work, make sure you don’t have any firewall or permissions apps preventing the Play Store from connecting to the internet.