You should be able to. Get one that can support the startup amp draw of the compressor. I'd say a 20amp timer should be plenty for most portable AC units, but I'd check the manual and base my sizing off of that.
The A/C systems that I have used before have an issue with a full power shut-out. They reset to default settings when unplugged (basically what your doing by putting it on a timer). You may not be able to do this but with mid- high range a/c systems they should have built in timers.
I've started useing a dehumidifier.. it has digital controlls and
it restarts off a timer... it does advertise 'auto-restart'.
The portable AC, which does not 'restart' from a timer, has a 'reset' button/box on the plug..
The dehumidifier has no 'reset' button/box at its' plug;
if I were to cut the 'reset' box off the ACs plug and put regular male plug, might that allow restarting?