"Καλησπέρα, ευχαριστώ."

Translation: Good evening, thank you.

[EMILIA_MARIE]

good afternoon, thanks

[fotiosgaridis]

Good afternoon = Καλό απόγευμα

[Lng52-._]

Can you also say: "Καλησπέρα, σας ευχαριστώ?

[uEvc3]

That's the formal way of saying it. Informally you can drop the σας.

[fotiosgaridis]

Now it's accepted! Thanks for your feedback.

[chrisflorey]

Not an English phrase. What does this mean to Greeks?

[fotiosgaridis]

Many of the Duolingo sentences/phrases are not contextualised hence one needs to add an imaginary context around the sentence. I can imagine a lot of occasions where this phrase can fit, for example when entering in a building/fair/party someone offers you a leaflet/drink/coupons, etc. and you use this sentence.
Body language adds a lot of our day to day contexts and this cannot be represented always in an online app like duolingo.