The package it should install is qt5-qtbase-devel, but currently "kdesrc-build --initial-setup" apparently doesn't install that.
Can you please submit a merge request at https://invent.kde.org/sdk/kdesrc-build/-/merge_requests/ The relevant file is modules/ksb/FirstRun.pm. Don't forget to put "BUG: 437609" on its own line somewhere in the commit message!
A possibly relevant merge request was started @ https://invent.kde.org/sdk/kdesrc-build/-/merge_requests/106
Git commit c299f9d64540c7a556763d4502ae98cfde4d8fdb by Alexander Robb. Committed on 27/05/2021 at 12:47. Pushed by ngraham into branch 'master'. FirstRun.pm: Add qt5-qtbase-devel to Fedora packages M +1 -0 modules/ksb/FirstRun.pm https://invent.kde.org/sdk/kdesrc-build/commit/c299f9d64540c7a556763d4502ae98cfde4d8fdb