Error code 13 - Sony MDS-JE480 Mini Disc Player

I checked the manual and it says if you are recording the disc is dirty and cannot be read, if you are trying to play a disc the same may be true and to try and reinsert the disc.
I've only had 2 bad minidiscs out of hundreds of them, it is possible an old one has damage or dirt. It may also be the complex play and recording heads are worn out or damaged.
If you have dropped the unit the heads may be messed up. Most people don't understand MD's are read write and can be used many times. On the other head they record with a magnetic skate riding one side of the disc in the cartridge while a laser is firing at high power on the other side to record. They do not work like a CD recorder.
One will find many old MD decks with bad heads which will still play recorded discs but cannot record and save to a disc. To figure out if one is working try to record and save, after doing so eject and reinsert the MD if it is blank the recording head is shot regardless of what the unit says.
Most MD decks unless you have the upper end $1,500 ones most people have never seen are not worth the expense of repairing due to head cost and limited numbers of qualified people to do the work. In those cases it is better to just find another which works.