Typically the 5B00 error code means the ink absorber pads are full. The absorber pads collect ink that is, for instance, drained during maintenance purges of the cartridges. The printer keeps track of ink waste and when the counter reaches its limit then the 5B00 error code occurs.
Uou need to reset the ink absorber pad counter to clear the error code. To do this you will need to set the printer into Service Mode where you can then see the internal counters that the printer keeps track of. With older printer models you could enter Service Mode with button pushes on the printer counsel. Most likely you will need a Service Tool to access Service Mode.
You will need to use the proper Service Tool or you might "brick" the printer and will not be able to access Service Mode even if you get the proper Service Tool.
Older Service tools may be found for free on the internet and maybe on this forum. Newer ones can be purchased off eBay for a few dollars. There should be a list of printer models that the Service Tool works on so you can choose the proper tool.
The forum's Moderator @The Hat has generic instructions on using a Service Tool:
I found a video that shows the procedure for your printer model using Service Tool ST-5103. Searching Google for this service tool brought up several eBay pages selling it and listing your printer model as compatible.
The next thing is what to do with the "full" ink absorbers? You can do either:
Take apart the printer to get to the absorber pads and then wash out the pads (very messy!) and reinstall when nearly dry. Dry pads will not be stretchable to fit properly. Make sure to photo or video every step of the dissemble and number the parts as you go as there are many steps and alignment of parts to accomplish on the rebuild.
The pads may have room for more waste ink, maybe up to another "full" ink counter's worth. Place the printer on a tray that can catch overflow ink.
Depending a lot on the conditions of the place where the printer had been kept, the volatile parts of the residual ink will evaporate at a different rate. A hot and dry environment will favour evaporation and you'll have more room for additional waste ink before the pads overflow for real. Just in case, follow @stratman's advice about the 'safety tray'
The Canon service tools only work with Windows, you cannot use MacOS or Linux. The service tools only work with a USB connection even if the printer might have wireless capability. For the service tool to work without the risk of blocking access to service mode it is important that the printer is installed with USB.
This post is about the service tool v3400 but the procedure for entering service mode and resetting the absorber counter is the same for the v4720.