There are many reasons for GPS reception to be weak or even disabled.
When you first use the app, a pop-up will appear requesting permission for Karta GPS to access your location. Be sure to allow it, otherwise you won’t be able to navigate anywhere.
You can also go to the main Map and tap the My current location button on the right bottom corner. If it relocates you to your current position and appears active it means your GPS is working fine. If a pop-up message appears asking permission to use your location, you should allow it and then try pressing the My current location button again.
If you still can’t get a GPS signal, chances are that the cause might be due to external factors such as proximity to tall buildings, low-level spaces or even a malfunction on your phone’s GPS antenna.