In english you wouldn't have the article in this situation
Is that a UK thing? In New Zealand you would use the article.
Ditto U.S. English.
Not in the part of NZ that I'm from!
Whether you would use an article in English surely depends on the context - but Duo accepts either version so there's not much point in pushing your preference.
"They didn't go on their honeymoon" not accepted. Who else's honeymoon would they have gone on?
"Their" is not in the French.
In UK English you would also say "They didn't go on honeymoon"
"..did not leave for.." should be accepted. Right?
Yes, "They didn't leave on/for a honey moon" is fine.
Good to know :-) Had reported to Duo, no reply yet...
Yup, "didn't go on honeymoon" should also be accepted (UK)
It is accepted.
"They have not left on a honeymoon" seems to be the meaning of this, which is not the same meaning as the suggested translation.