What is a notational error in this measure? It is in common time and bass clef. I was thinking that the dotted half note is the problem, but I am not sure how to change it exactly. Maybe to a quarter note + half note?
It is mathematically correct, but it tells the world (or at least, people who care about the details of notation) that whoever did it doesn't know much about music engraving, and presumably isn't using software that automatically does such things correctly.
A rest like that should always be written to show the two halves of a 4/4 bar - in other words, a quarter note rest followed by a half note.
For reference, here's page 161 of Behind Bars by Elaine Gould, generally regarded as the "bible" for modern music notation.
Aside from that specific rule, writing dotted rests is generally discouraged except when they start a beat, because in complicated rhythms it can be hard to keep track of the dots. Which of these is easier to read? The second bar shows the four beats in the bar, and subdivides them into 8 half-beats to save the reader having to think about how to count the rhythm. The first bar doesn't.
The rest really would be best to be written as a quarter note rest and then a half note rest. As a band director, I don't think I have ever seen a dotted half note used like this. Very rarely are dotted half note rests used
There is no error. The note shown is worth one beat. The rest is worth two beats, and with the dot is worth half that again. 1+3 =4. Job done. What is the question about?
This doesn't look wrong to me.
However - is it the final bar of the piece?
If so then you need to look at the start of the piece to see if it started with a complete bar. If it did not (i.e. it started with a partial bar forming an anacrusis) then the length of the final bar is supposed to reflect that and the two together should add up to one complete bar. You say that it is in common time so for example if there was a one beat anacrusis at the start then the final bar should contain only three beats to make a total of four.
Apart from that possibility I can't see the issue.
As for the time, you have a (dotted half rest) = (3 beats) and a (quarter note chord) = (1 beat) for a total of 4 beats. So the accounting works, no problem.