Why can't I place a rest in a 4/4 time signature before 3 quarter notes?

For some reason (do you know which one, voluntary or not?), the duration of the measure has been shortened by one beat (hence the small minus “-” above the right-hand side of the measure).
See how to do this (GIF below).

Alternatively, after restoring the nominal duration, you can delete the contents of the measure and re-enter the notes. This is quick for three notes. But you'll also know that cutting and pasting is the solution recommended by MuseScore.
One last thing: to select a range of notes (e.g. three notes), click on the first, then Shift + click on the last.

Your picture shows a non-metered measure:

From the handbook:
"If, at any time, the total duration of the notes and rests within the measure does not match the time signature, a small + or - sign will be shown above the measure."
See:
https://musescore.org/en/handbook/4/pickup-and-non-metered-measures#non…

Also:
https://musescore.org/en/handbook/4/measure-properties#dialog
especially:
https://musescore.org/en/handbook/4/measure-properties#measure-duration

> and why is the "Ti" or "B" Quarter Note is color red?
I don't know which instrument is on this staff, but it usually says that this note is lower than the instrument can play.