Like the error message says, you need to use the "factory image" instead of the sysupgrade image. You should be able to flash it by selecting the "force upgrade" there. And unselect the "keep settings".
The underlying reason is explained in that error message that you see: Linksys had defined too small partition for the new kernel versions (used in 21.02 and master) for WRT1900ACv1 (mamba) and WRT32X (venom). So, the kernel partition has been enlarged for those two, but that prevents using the sysupgrade, and you need to use the "factory" image to start fresh.
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=8458ebe18b84702039020c523018e39f564ccf6c;hp=7939d4a1b13b0c44dbfa4cd03180fe4cd5b2fe6b
At the same time you also get the switch config change from swconfig to the new DSA, so you should not restore your old config from backup...