P.s.: in a review of this mainboard you will find two important notes: supposedly the USB stick for the BIOS update must have a USB 2.0 and be under 8G. In addition, the file name must be written in capital letters, as described in the instructions (this should already be clear if the file name is written like this in the instructions).

As already mentioned, you can update the BIOS with a USB stick and the mainboard without any components installed. To do this, however, you must follow the instructions down to the last detail. For example, the stick must have a FAT32 file system. If this still doesn't work, I'd also advise you to build with an older CPU, with which you can then boot and update the BIOS directly from the BIOS menu. I have a 5900X Problemos running in the board, but I bought a 2700X for the waiting time of the CPU and then installed the update before the delivery of the 5900X.
Is it possible that you are doing the update procedure with the stick with the 5000 series CPU installed? In this case, I would advise you to remove the incompatible CPU first and then do the update again, as the mainboard may recognise the incompatible CPU before the update is initialised and block everything.
Also: I wouldn't recommend installing the very last BIOS, as in most cases this is a beta, install the second to last update without beta, works perfectly with 5000 series!

Im Manual auf Seite 32 ist beschrieben, wie man das BIOS mit dem USB Stick updated. Das ist nötig für die neuste Ryzen Generation.

Manual:
https://download.msi.com/archive...
BIOS Download:
https://www.msi.com/Motherboar...

1. Please download the latest BIOS file that matches your motherboard model from the MSI® website.
2. Rename the BIOS file to MSI.ROM, and save it to the root of your USB flash drive (FAT32 format).
3. Connect the power supply to CPU_PWR1 and ATX_PWR1. (No need to install CPU and memory.)
4. Plug the USB flash drive that contains the MSI.ROM file into the Flash BIOS Porton the rear I/O panel.
5. Press the Flash BIOS Button to flash BIOS, and the LED starts flashing.
6. The LED will be turned off when the process is completed.

FAT32 ist ein muss, sonst updates nicht.

I flashed the bios with a stick. Not the latest version, a previous one. There are YouTube videos about it.

Otherwise, try a CPu from the last generation. Maybe you can borrow one from a friend or something for a short time. (I don't know if it's ideal, but it's better than nothing...) If that doesn't help, maybe check with Digitec Support and, in the worst case, ask STEG, they have various services in this area - maybe they could help you there.