At the first start of the app, it asks for permission to use the location of your phone. Please make sure that you have granted that permission - you can find an app's permission in your phone settings. Alternatively, you can uninstall and reinstall the app and grant permission - no worries, no user account data or observations are lost, you have simply to log in again.
If the reticle doesn't jump to your current location, you can also use the address search to find it or move the map to your current location manually.
If you want to be use your location, you need to turn on "Location" in your phone's quick settings (if it wasn't turned on already) and open the app again, wait a few seconds and click on the crosshairs