Hello Nanatan93,
We understand that you may be experiencing a problem using the App Store or otherwise attempting to sign into your Apple ID account and are happy to assist.
To begin, are you able to sign into your Apple ID account using your credentials at Manage your Apple ID ? If not, following the links on that page for Forgot Apple ID or password? would be appropriate to regain access to your account.
If you can sign in there, the next element to isolate are the Wi-Fi connection that you are using on your device. Try changing to a different Wi-Fi network or attempt through a cellular data connection to see if this error continues. Disabling Low Power Mode per Use Low Power Mode to save battery life on your iPhone - Apple Support can also be tested for relevancy to this error.
We hope this helps!