"He has a blue flower."
Translation:Li havas bluan floron.
It's not the most common word order, and so it the maintainers haven't added it as an alternative yet, but it's grammatically correct. You can report it if you want.
Esperanto's flexibility means that an English sentence can often have many possible, valid Esperanto translations, and maintainers can't think of them all beforehand and add them as accepted alternatives. Instead, they rely partly on reports from learners, especially since Esperanto is still in beta.
They do end with -o and -a, it's just that they have an -n even after that so that the -o/-a is no longer right at the end here.
This is the accusative case, used for direct objects of verbs. What does he have? A blue flower.
So you have blu-a-n flor-o-n.
If the blue flower were in the nominative case (for example, if it's the subject), then you don't have any case ending and then the part-of-speech vowel does come right at the end: "Blua floro kreskas sub la arbo" (a blue flower grows under the tree).
Note that the plural marker -j also comes after the part-of-speech marker (and before the accusative marker, if present), so you might also see "bluaj floroj" or "bluajn florojn".