Downloaded the latest (5/31/21) nightly (just SimutransExtended64.exe and the Pak128Britain set) from the relevant sticky and I can't make a schedule into a line or adjust the timings. App version says 120.4.14.14
The "promote schedule to line" button exists, but it does nothing. Additionally, if I make a line duplicating the schedule and assign the vehicle to it, the vehicle does not "follow" the line (i.e. it doesn't show up under the line under line management and doesn't show anything in the Line area in its info screen).
For setting timings, the "wait for schedule" option is grayed out, I can't even select it. The line is valid and "works", and the service frequency data is filled.
Thanks!
This was fixed a few minutes before your post (see https://forum.simutrans.com/index.php/topic,20932.0.html), and probably did not have enough time to propagate to nightly. As of this writing and the latest version from github, it is working whilst compiled on linux.