Something must have went wrong when you tried to install Matrix to internal.
Please follow these steps:
- (re-) flash Android firmware (ensure it is version 7 or higher)
- prep external media (USB Flash Drive, SD Card) to boot CE
- boot CE from external media via one of many methods (toothpick, terminal command,…)
- use ceemmc to install CE Matrix to internal.
Be aware installing CE to internal storage is not officially supported by Team CoreELEC.