What does it mean when the priinter says waste tank is full

Follow the below mentioned steps :
1.
Remove the outside case of your printer.
2.

The Printer Body on the Base
Once the case is off, you're looking at the guts of the printer sitting in the tray-like base. To get at the blotters and waste ink, you'll need to get under the printer body, which is held to the base by three or four screws. The older your printer is, the easier it will be to find the screws that hold the body to the base, and the more working room you'll have. If you have a good magnetic screwdriver, that will be very helpful. It may also be helpful to take notes and/or pictures of where the screws are located to make re-assembly easier.
3.

One of the Rear Corner Screws
Use your flashlight or lamp to illuminate the rear corners of the printer base. You should see a small screw on each side with an access for your screwdriver to it - remove each of these screws, and the body of the printer should be freed-up from the rear of the case. Now look for one, or usually, two more small screws on each side of the case bottom toward the front. With the removal of these screws, the printer body should be able to be raised at the rear and lifted backward out of the case. Some models have a hidden screw or two, which might even require the removal of the power supply, or the movement of wire assemblies to get at the screw. But there always will be a way.
4.
Put on a pair of latex gloves. With the printer body removed from the case, and set aside on some newspaper (it may/will be very "inky" on the bottom!), you are now looking at the blotters which have received the excess ink - see the top pic. Again, it's a good idea to take notes or pictures to indicate the location of the blotters. There are usually several large and small ones. Remove them to plenty of newspaper and take them to a utility sink, idealy in your garage or the utility room (I use our kitchen stainless steel sink, but my wife is very gracious and I'm VERY careful!).
5.
The cleaning of the blotters will take huge amounts of running water! Hold the blotter under a stream of running water as it flushes the waste ink out. This will take a long time - maybe 10-15 minutes for each one. No, you will never get them white again, but your goal is to remove enough of the waste ink that eventually, the wash water is just slightly inky and the blotters are grey. You may gently squeeze them as you wash them, but this may damage the blotters, so go gently. When you decide they've had enough cleaning, let them swell with warm water until nice and plump, and do not squeeze this water out. Put them outside in the full sun on some thick newspaper to fully dry - occasionally turning them over. If it's raining in your area, you may choose to do the drying in your oven - use only the oven light, which should push the temp to near/over 100 degrees, and lots of newspaper.
6.
While the blotters dry, take the printer tray/base to the sink and give it a good cleaning. Make up a Clorox/water solution to help remove ink stains from the base and the outside of the case too. When the blotters have dried - two days is not unusual - you're ready to begin the re-assembly of the printer. Use your notes/pictures if you've been smart enough to have them. Replace the blotters. The printer body will slip into the bottom of the case from the back toward the front. Replace the screws. Remember to re-route any wiring, and to reconnect plug-ins. It's a good idea to power it up before you put the case back on, just to make sure all is well. Replace the case, and power it up.
7.
If you're lucky, you will no longer see the flashing error code lights (they are different on many models, but usually on the Pixmas, you'll see a sequence of 8 orange flashes followed by one green). If you are still seeing this error code flashing, you'll need to reset the printer's internal setting - to do that, see the last link in the Resources below titled, "How to Fix the Dreaded Waste Tank Filled Error Message for Your Canon Printer". And if you continue to have issues, Please reply back for assistance.