My door won't lock on my Kenmore he washer leaving the message F dl. What can I do?

You need to adjust the door latch, and possibly the door switch, too. This is an easy job to do. Press in on each side with a small putty knife between the top and bottom section on left and right sides to release the spring clips that hold the top down. Then you can get to the latch. Look in this manual for instructions: https://dl.dropboxusercontent.com/u/8010163/Kenmore_Washer_Elite.pdf
After you get the latch adjusted so that it latches well make sure to check the micro-switch to make sure it's opening and closing the contacts as it should. With machine unplugged (as it should be before you do any work on it) unplug one wire on switch (try it first with the wires plugged on it and it will probably give you enough of a reading for you to know if it's working) and put the leads of an ohm meter on each terminal where the wires are plugged onto it. Note if the reading goes from zero to a high reading, with wires on, or infinity with one wire off. If it does this as door is opened and closed the switch is adjusted okay. Alternately, you can listen for a click on the micro-switch or feel it with your finger. You can feel it snap open and closed if your finger is sensitive enough. That's all you need to do. These doors get out of adjustment from time to time. Make sure to tighten the screws well by using the largest and best screwdriver you can get that fits the screw. If it's a hex head with a slot, use the hex head nutdriver or it will not tighten enough and you may not be able to loosen it without ruining the head. Good luck with the repair.