Sysupgrade error (from 18.06, ar71xx to ath79) - Installing and Using OpenWrt - OpenWrt Forum
I also encountered a similar problem before, but I upgraded successfully after using tftp mode. If you are not using the firmware version with the ...squashfs-factory.img suffix, I suggest you use this version and try to upgrade in tftp mode. Because the version before 19 and the version after 21 may be different in terms of partitions