You’re using the wrong firmware. Keep trying with different firmware till you find one that works.
Also please be aware that as per this article we do not provide official support for this situation:
Unsupported devices:
With the parameter x the ceemmc tool can be used for not by Team CoreELEC official supported devices.
Any support request for not official supported devices will be ignored.
This is because Team CoreELEC does not own every device on the market available and so we can not confirm the tool function with the unsupported device!