DIAGNOSIS That's a non-resettable maintenance counter. Inside the printer is a felt pad that soaks up a tiny bit of waste ink from cleaning cycles. The printer estimates this pad is saturated. It's a built-in counter, not a physical sensor.
FIX For the ET-2800, this message means the printer has reached a maintenance point set by Epson. You cannot reset this yourself through a button combo or menu. The printer will eventually stop working. You need to take it to an Epson-authorized service center. They have the proprietary software to reset the waste ink counter after inspecting or replacing the pad.
IF THAT DIDN'T WORK There is no user-serviceable 'fix' for this. Beware of online videos showing secret button presses—they are often for different Epson models and won't work on the ET-2800, and can lock your printer further.
HEADS UP This is based on estimated ink usage, not actual pad saturation. If you've used a lot of power cleaning cycles, it triggers sooner. Contact Epson support directly; sometimes they provide a one-time remote reset code if the printer is relatively new, but don't count on it.