What does this warning mean?
# acis_process_events (CIAO 4.2): WARNING: problem reading ctifile, cti adjustment will not be applied. Changing apply_cti=yes to apply_cti=no.
The most common cause for seeing this warning is that the observation does not contain -120C data. Since the CTI correction is primarily calibrated for this focal place temperature, it is not possible to use it on other observations. Note that this is not an error; if no CTI file is found, acis_process_events continues running as if apply_cti=no and produces a valid output file that has not been CTI-corrected.
The CTI Correction why topic has more information on the correction.