Yep. The build file is wrong, the very last number points to version (bunchofnumbers)11 instead of (bunchofnumbers)22 which is the name of the actual file.
curl: (22) The requested URL returned error: 404 Not Found
==> ERROR: Failure while downloading http://ppa.launchpad.net/system76/pop/ubuntu/pool/main/p/pop-gtk-theme/pop-gtk-theme_5.2.0~1598539187~20.04~665ad11_all.deb