Error code P-21 showing on the screen, printing picking up cyan background. What is reponsible?

First check the Laser window cleanliness,
Then check the IU-Cyan and its charging grid cleanliness.

also try in service mode-Image Processing Adjustment-
Background voltage value can be increased.

Recommend you buy a OEM cartridge cyan

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/normanj_7893313f0751935f

brother dpc 1510 printer showing on print unable 0B. how to clear?

What are you printing a Word document when this happens ?
suspect pattern may be in the software you are printing from.
Go to software help screen and look search for removing background pattern or removing watermark.

Get in touch with the friendly Xerox Engineer, I think I remember this error code being the Cyan print head is not performing as it should. A bad batch of Cyan was allowed onto the market a lot time back and the plumbing and print heads became bunged up, due to crystallisation within the hot wax.
Hope you have a service contract!
Good Luck.

anyone no idea?

Yes you can "recharge" the developer. It involves removing the cyan developer assembly and replacing the cyan developer powder.

Check the black clip on cover on top of the paper pick up assembly under the cyan cartridge.

If it is not fitted correctly cyan will not print and no error message will be displayed.

Check the black cover on the pick up assembly under the cyan cartridge. If it is only a fraction too high cyan will not print. Make sure it is clipped in place correctly.