# acis_process_events (CIAO 3.4): WARNING: No files found matching 
CALDB search:
        tel=CHANDRA
        inst=ACIS
        det=-
        filt=-
        product=CTI
        start_date=1999-08-20T00:07:44
        start_time=00:07:44
        stop_date=1999-08-20T02:26:38
        stop_time=02:26:38
        query=-
# acis_process_events (CIAO 3.4): WARNING: PROBLEM READING CTIFILE, CTI 
ADJUSTMENT WILL NOT BE APPLIED.

The most common cause for seeing this warning is that the observation does not contain -120C data. Since the CTI correction is only 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 Apply the ACIS CTI Correction thread and why topic have more information on the correction.