THIS is the kind of sentence that the unit SHOULD have begun with; Part 1, not Part 8!!
Reinforce the simple phrasing and we can learn the patterns. Ho deciso. Ho mangiato. Ho finito.
Then move on to "tu", and "è", and "loro", etc.
THEN go on to complex sentences, and idioms like "suonato alla porta".
I put "I made up my mind" and it wasn't accepted, but around here it's an expression that is very commonly used as a synonym for "I have decided".
I must admit I do like to vary my answers a bit from what I think Duolingo is expecting, just to see how flexible it is getting over time. In this case, not very.