Can't get the auto pilot to engage on the Iflyjets 737NG

Does this help, I found it elsewhere.
If not contact the company that you bought it from..

A paid copy of FSUIPC will end your problems. It has a setting that centers the trim when using the AP, to prevent your problem. From the manual:

Centre elevator on AP Alt change makes FSUIPC4 centre (zero) the elevator input when the Autopilot ALT hold mode
is enabled or disabled. This is mainly of use for keyboard flyers, where the original keyboard-set value for the elevator is
retained, ineffectively, whilst the A/P controls the vertical modes, but which then undesirably re-asserts itself when the
A/P control is relinquished. See also the note in box below.

Disconnect elevator trim axis for A/P: This option is aimed at those who are using an analogue trim axis instead of the
more usual trim up/down buttons, wheels or levers. When the FS autopilot has control vertically (such as in altitude hold
or glideslope acquired modes), it disconnects the elevator axis so that joystick inputs don’t upset things, but doesn’t do the
same with the trim axis. This FSUIPC4 option fixes that. If it is enabled it also disconnects the trim axis if an add-on or
external autopilot disconnects the elevator axis via the special