Ubuntu MATE 20 04 not installing on RPI 400

I am not getting to the installer or anything, it just says no card detected and i´m using a new sd card

Did you use the dedicated Raspberry Pi Imager to flash the USB stick? Sometimes that works better than Balena Etcher. Rufus has been my favorite imager.

I used the RPI Imager.

Hi again simeona. Just checking to make sure you downloaded the ARM 64-bit version for Raspberry Pi. I should have asked that the first time - I was on my iPad and was not thinking. Desktop PC 64-bit won't work on ARM. Just checking. If you are using the correct download with the proper imager, try a different SD card. I had that once whereby I had to reformat the SD card and even though I could correctly flash an SD card with the correct image, it would not boot. I had to use a different SD card until I found an application for Windows PCs called SD Card Formatter that does wonders on SD cards and USB sticks. When I have an issue with booting or flashing, I reformat the SD card or USB using that app and suddenly I can use it again for a fresh install. See links below...your mileage may vary. :v: