Possible Causes
You are in Mapping Mode rather than Route Mode while attempting to spray.
RTK is turned on in the Drone Settings while trying to use GPS
Issue with RTK Base Station
Solution
If you are attempting to spray, ensure you are in Route Mode, not Mapping Mode. Note that Mapping Mode requires RTK to initiate takeoff
To select the correct mode, click on the square in the top left corner of the screen
If you do NOT want to use RTK and only use GPS:
Make sure RTK is toggled off. This is located in the drone settings (Top right-hand corner gear icon), then RTK Tab. Make sure the first toggle is off, when it is the menu disappears. Please note this is the most common cause of this issue as after drone firmware updates, this setting can be turn on automatically.
If step 1a doesn’t resolve the issue, power off the drone and remove the battery for 3 minutes. T40 models and newer retain some power to maintain the RTK signal during battery changes, so this step is necessary for a full reset of the drone.
Check to make sure the drone is not close to powerlines or cell towers, if so move the drone away and test again
Check Kp values for local area, strong magnetic solar storms can cause interference from drone to satellite signal. If K values are high in your area, you will need to wait till the solar storm passes.
A good website to check Kp values is UAV Forecast, a mobile app version is also available
If you ARE using RTK with a DJI RTK Base Station
Make sure Base Station is setup properly, see more details in RTK Base Station General Setup