Whirlpool Duet model WFW9451XW00: Replace motor control unit to try and fix F28 code, but that did not work. What should be the next step. Thanks very much.

The F28 error is a communication error between the central control unit and the user interface control boards. In most cases it's the central control unit that needs replacing. The control you need can be ordered at the link below.

https://prismparts.com/products/washer-electronic-control-ccu-central-control-unit-part-rp9222206paz63