https://www.thoughtco.com/unable-to-read-your-obd-ii-codes-281614

On many cars, the ECM (that's the electronic brain or computer) is on the same fuse circuit as other electrics like the cigarette lighter / accessory port. The lighter is prone to blow fuses on some vehicles, and if there's no juice going to the ECM, it can't tell you what's wrong. Even a fuse that is dedicated to the car's computer diagnostics can blow for no apparent reason. The biggest cause for getting no OBD code at all is a blown fuse. Check your fuses to be sure none of them have gone bad. Keep in mind, too, that your car or truck may have more than one fuse box. This should be covered in your owner's manual or a proper service manual.

From time to time, the scan port can become clogged with dust from years of being unused. You would never want to spray a cleaner or get the port wet, but wiping it with a soft cloth or blowing some compressed air across it can help clear out anything that may be preventing your scan tool from getting a good reading.