The first thing you should do is check all your connections, including that the cables' threaded connections are tight, and that the accelerometer is firmly attached. Then reboot Windows on your PC. This should clear the error.
If you repeatedly get a screen that looks like this with an endmill larger than 3/8" and smaller than 1", it may be the indication of a bad cable. Most likely it is the thin accelerometer cable as the hammer cable is very robust. You can also tell which cable it might be by the location of the red arrow. If it is in the upper window it is the hammer cable. If it is in the lower window it is the accelerometer cable.
If you are tapping a very large solid carbide endmill (1-1/4" or larger) or a big indexable endmill (2" or larger), the hammer blow is not making it across to the accelerometer. Go back to the beginning, add the mass weight to the hammer and select "Indexable Face Mill". This will get the proper settings. You can enter the correct tool (doesn't have to be a face mill).
You can run a cable test yourself using MillMax. You will need to go to the home page for MillMax by hitting F1 or the TXF tab in the far upper right corner of your screen. Then press the CTRL and A keys at the same time to bring up the SpeedCast advanced menu. Follow the instructions on this video:
If you need to replace the cable, be very careful to when threading the accelerometer and the BNC adapter making sure they are fully threaded together and finger tight.
For very small or very large cutters, a Hit Harder message could mean that you need to go to the metal tip (in the case of small endmills) or add the hammer mass weight for large cutters. If your ables are good and you are still getting "Hit Harder" errors, try this: