'Excuse' could be used as a noun or a verb, and I think that's the confusion here.
As a noun, we say 'Make/have an excuse for something'. As in, "She had an excuse for not doing her homework."
As a noun, 'excuse' means 'reason'.
As a verb, 'to excuse (from)' means 'to allow to leave' OR 'to allow not to do'.
'Excuse of' means 'to forgive'. This is a bit confusing but let's look at an example.
'The teacher excused me (from the class)' means that the teacher allowed me to leave class.
'The teacher excused me from coming to class' means that the teacher said I didn't have to come to class.
'The teacher excused me of being late' means that the teacher forgave me for being late, or maybe that the teacher doesn't really think I was late at all.
I hope this helps! I know these little differences can be frustrating, but keep asking questions!