Hello @jbolanosg and welcome to the forum.
Can you share the dtb file you used?
There is emmc installer file already present in the installation. No need for manjaro-arm-installer that is not the right way for Amlogic boards.
Just make sure you take backup of your content of emmc.
start Manjaro with latest image over sd card.
Run this command
sudo /boot/install-aml-emmc.sh
Make sure your date time is correct before running this command.
Once it show Flasing Manjaro to eMMC Completed then restart the device with sd card inserted and reboot it 2 times with Manjaro on SD card after that you can power off and remove the sd card and power it on with Manjaro on eMMC.
Try this as your own risk as we have not tested this on your device. I or Manjaro Project will not be responsible if you brick your device.
Good Luck.
Tutorial Link Up: