There’s some natural complexity to trying to build a project from git using the latest libraries also built from git. It can be rather overwhelming at first.
Wayland should have been fixed by the upstream MR by now, the fix was merged yesterday so it should build correctly now. In addition to the things mentioned in Can't build third-party/wayland, you can also skip building any problematic dependencies and make kde-builder just use your distribution packages for those projects.
If plasma-login-manager is the thing you are trying to build and contribute to, since the error you got seemed to be an actual source code issue, you can ask questions about the project and how to fix this on Matrix - KDE Community Wiki (unless some plasma-login developer shows up here).