AUR expects unstable branch and as you likely recall - totally unsupported.
Talk to the developer … it is one of the tests failing
Perhaps a dependency is missing - I suggest you try with pamac instead of yay
$ pamac build python-caldav
[...]
==> Finished making: python-caldav 1.3.9-1 (tir 02 jan 2024 14:07:49 CET)
==> Cleaning up...
Checking keyring... [1/1]
Checking integrity... [1/1]
Loading packages files... [1/1]
Checking file conflicts... [1/1]
Checking available disk space... [1/1]
Installing python-caldav (1.3.9-1)... [1/1]
Running post-transaction hooks...
Arming ConditionNeedsUpdate... [1/2]
Refreshing PackageKit... [2/2]
Transaction successfully finished.