I compiled the latest 19.07.4 OpenWrt for the TPLink TL-WDR3600 and the build appears to have finished with no errors. I did not make any changes to the default menuconfig except setting the below:
A squashfs factory and upgrade images were generated in the the directory bin/targets/ar71xx/generic/.
When I upload either of the bin files through Luci it shows two errors:
Image metadata not found. Use sysupgrade -F ...
The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform.
The factory and upgrade bin files built are parsed as 7.75mb and 4.25mb by Luci.
Does this mean the image did not build correctly or is this a normal thing for self-built images? Thanks.
Sounds like you built a deprecated ar71xx image, while your router may already be running the more modern ath79 target image. And that causes the error.
ar71xx is deprecated.
You should build from the ath79 target.
(And as you are upgrading a router already running OpenWrt, you need to use the sysupgrade image.
Factory is used if you flash from OEM firmware or recovery)
Ps.
The upgrade path from ar71xx to ath79 has usually been configured to be smooth, but trying a downgrade back to ar71xx causes that compatibility error. You may override the warning in LuCI, but I think that you should stick to ath79 based firmware.