This can be caused by a bad accelerometer(in the computer) or a pitch servo. Most likely it is an accelerometer. The aircraft should not be moving during the PFT. Attached below is from the standard pilots guide.

A preflight test is performed upon power application to the computer.
This test is a sequence of internal checks that validate proper
system operation prior to allowing autopilot engagement. The preflight
test (PFT) sequence is indicated by “PFT” with an increasing number for the sequence steps. Successful completion of self test is identified by all display segments being illuminated (Display Test) and the disconnect tone sounding.

For two-axis units only:
NOTE: Following the preflight test, the red P warning on the face of the autopilot may illuminate indicating that the pitch axis cannot be
engaged. This condition should be temporary, lasting no more than 30
seconds. The P will extinguish and normal operation will be available.
NOTE: The red P warning may illuminate when the autopilot is not
engaged. This can occur when autopilot G limits have been exceeded during turbulence or aircraft maneuvering. Autopilot engagement is locked out during red P illumination. If power to the autopilot is cycled in flight (i.e. through the autopilot circuit breaker for instance) it is possible that a 5 minute delay may be necessary prior to autopilot engagement to allow the pitch axis accelerometer circuit to stabilize. Engagement prior to stabilization may result in mildly
erratic pitch axis behavior.