Netgear r6220 upgrade problem

So I compiled a brand new v21.02.0-rc3 firmware for my Netgear R6220 but when I tried to flash it I got this message:

Flash Firmware LuCI.png

Are there two problems? The first may be a simple typo and easy to resolve, although I don't know how.

But what about file format not supported? And that config will be wiped? I don't want to brick it as it is the only internet gateway I have... What to do?

This is a totaly normal behavior.
Error message means that you are running a 19.07 using swconfig, while 21.02 uses DSA, one of the main feature for the new version. It is safe to force upgraded. After upgrading, go to network, interfaces, and Luci will migrate your config.
However, if your config can be done quickly, I would recommend not to keep settings and so start from a fresh install while upgrading. It's up your choice.

Problem is, if it doesn't keep the config LAN will start on and wireless won't at all, so I may be left without LuCI...

I am not going ahead with the flashing procedure because the warming basically says that I'm going to brick the device due to an unsupported file format. That's clearly not right.

How can I create a file format that is supported with v21.02.0-rc3?

This error message is normal.
It is meant to inform the user for an incompatibility of settings while upgrading.

19.07 and 21.02 use different program for switch managment, so there maybe settings issues while upgrading. 21.02.0.rc3 includes conversion mechanism in luci. So it is safe to upgrade. I have done it many times with this device and several others too.

The image builder provides image file that work just fine.