If you are using a zip with the Play Store, this is because the author of NanoMod has patched the Play Store apk to prevent the app from crashing when you try to make purchases on Google Play. If you are using FakeStore, it's because you aren't using the Play Store.
To fix the issue for both apps, open a terminal on your phone or enter
adb shell on a computer and enter the following commands to gain root access and manually grant the signature spoofing permission:
su pm grant com.android.vending android.permission.FAKE_PACKAGE_SIGNATURE
Close the Play Store if it is open and exit the microG “Self-Check” (if you haven't already), then view the Self-Check page again. It should report that the Play Store has the correct signature.