_____ _ _ _____ ____ / ____| | | | | ____|___ \ | (___ | |__ __ _ __| | _____ _| |__ __) | \___ \| '_ \ / _` |/ _` |/ _ \ \ /\ / /___ \ |__ < ____) | | | | (_| | (_| | (_) \ V V / ___) |___) | |_____/|_| |_|\__,_|\__,_|\___/ \_/\_/ |____/|____/
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.