Welcome nsbcr,
BTT products have usually a presentation on Github. GitHub - bigtreetech/Manta-E3EZ.
Most of the time there are others who ran into the same problem before. Did you try DFU?
You may read also, 'Printer is not ready' Error - #2 by mykepredko and Can't flash MCU persistently, via SD card or DFU · Issue #50 · bigtreetech/Manta-E3EZ · GitHub.
Good luck.