Translation:It was my intention from the beginning.
Just because your translation hasn't yet been suggested/accepted/programmed by Duolingo, doesn't mean that it isn't an appropriate one. I have had several suggestions accepted by DL over the past months. Every time we contact DL with an alternate interpretation and it is considered, accepted and added to the database, we have helped to improve DL. I encourage you to suggest yours.
I agree that the meaning is exactly the same and a quick check would show the Duo people that outset is a synonym of beginning. It is probably likely that when the computer was programmed 'outset' was not considered. If you care to, the next time you encounter this sentence, use outset and then when the response is that you are wrong you can use the button to then select 'my answer should be accepted'. The word may be added at some time in the future but probably not soon enough to help you. I believe they have trouble keeping up with all the comments and suggestions however I have had quite a number of words/phrases accepted over the past few years.
In your sentence 'since' should be 'from' so 'It was my intention from the beginning'. In order to use since, which indicates continuation, the verb form would change to 'It has been my intention since the beginning.' in which case the Spanish verb would change to 'ha sido' but that's not what Duo used. I hope this helps.
Thanks, now I've learned more English grammar! I never wanted to learn the intricacies of grammar in school. That's probably why I was a poor language student. Now I am happy to learn more English grammar and I am certainly finding it helpful as I am learning Spanish.