Bash script for check files

I created this script for check whether specific files exist or not in the given location. but when I run this its always showing

Failed - Flag_lms_device_info_20160628.txt do not exist
Failed - Flag_lms_weekly_usage_info_20160628.txt do not exist

but both files are existing. appreciate help for fix this.

Scripts -

PREFIX="/opt/lms/sftp/ewduser/sampledata/"
REPORT="/tmp/flg_file_status-it360.txt"
DATE=$( date -d "${dtd} -1 days" +'%Y%m%d' )

rm -f "$REPORT"

            FILENAME="Flag_lms_device_info_$DATE.txt"
            FULLFN="$PREFIX/$FILENAME"            
           
            if [ -f 'find "$FULLFN"' ];then
                    echo "OK - Flag_lms_device_info_$DATE.txt exists" >> $REPORT
            else
                    echo "Failed - Flag_lms_device_info_$DATE.txt doesn't exist" >> $REPORT
            fi
			
			FILENAME="/opt/data/Flag_lms_weekly_usage_info_$DATE.txt"
            FULLFN="$PREFIX/$FILENAME"           
           
            if [ -f 'find "$FULLFN"' ];then 
                    echo "OK - Flag_lms_weekly_usage_info_$DATE.txt exists" >> $REPORT
            else
                    echo "Failed - Flag_lms_weekly_usage_info_$DATE.txt doesn't exist" >> $REPORT
            fi

Why are you using "find" command in the conditions?
Check if file exists is easily done by simple using [-f "name_of_the_file"] structure.
Using condition your way you are asking if whatever output of 'find "$FULLFN"' is exists as a file, which is probably not what you want.

On top of what smoofy said, the single quotes used prevent any shell expansion. I guess you want "command substitution" to take place: replace the single quotes by - deprecated - backticks or - better! - the $(...) construct.
Two more thoughts/observations:

• Do you REALLY need to prefix the absolute second file name?
• Why do you define a shell variable and then don't use it in the echo statements?