This detects the accelerometer reading in the wrong direction. The most likely cause is that accelerometer is on the tool backwards, that is the unmarked side is opposite the tool surface.
If the accelerometer is mounted correctly and you still get this error message, check that the cable to the accelerometer is tight and that it is firmly mounted with wax and not loose on the tool.
This error can be caused by a "double-bounce" of the accelerometer. Similar to the double hit but only on the accelerometer side. First thing to try is use the hard tip on the hammer and strike lighter. This can often happen with long, large diameter cutters, both indexable and solid carbide endmills. To correct this, restart MillMax and choose "Indexable Face Mill" to change the hammer settings. Add the weight/mass to the hammer and select the correct tool (not a face mill) in the Setup screen.
UPDATE: This error may also appear due to a short in the accelerometer cable. Use this procedure to test the cable: https://mli.supportbee.io/2770/5986/18833