Firmware OpenWrt Install URL Not downloadable
For some reason, that device doesn't have a factory image download for 19.07.7 and higher (including 21.02.x). If you're currently using the stock firmware and want to install OpenWrt, you will need to use the factory image. The best way to do it in this case is to use the image from 19.07.6. Then, once you are on OpenWrt 19.07, you can use the sysupgrade image of 21.02.1 to get up to the latest. https://firmware-selector.openwrt.org/?version=19.07.6&target=ramips%2Fmt7620&id=tplink_c2-v1
In light of @mk24's comment, please ignore my advice... what I stated above will brick your device. Beyond that, I was accidentally looking at the C2, not the C20.
The C20v1 is not a model that can be directly installed from the factory GUI. Read the device page.
It will brick if the directions are not followed exactly. The "factory" file needs to have part of the actual factory firmware attached to it before submitting to the router. Since the actual factory firmware is copyrighted, the OpenWrt project cannot modify and redistribute it. It is necessary for the user to do that themselves.
The "firmware selector" should have a way to flag these models which are not as simple as finding a firmware and flashing.
Welcome to OpenWRT community.
In c20 v1, to install openwrt, you need to download the openwrt factory firmware and merge it with the uboot from official firmware, the resulted file can then be installed using TFTP. First installation can only be done by using TFTP. Then you can use the upgrade firmware file later on.
Follow the guide here carefully https://openwrt.org/toh/tp-link/archer_c20_v1. Failure to do so will result in a hard brick and would require programmer to recover from it.
Download factory firmware 19.07.6, merge the file with uboot partition from official firmware, then flash the merged file by TFTP.
Be extra careful, follow install guide above, any slight mistake will result in a bricked device.