Translation:Everything looks normal from the outside.
The verb is "aussehen" (=to look like)
It is a seperable verb, which means in its inflected form, the prefix gets detached and put on the very end of the sentence. This order, as always with verbs, is fixed and can't be moved around. The only other possibility is that you have another verb, which would take up the main spot in second position. In this case, the verb gets reunited and stands at the end of the sentence completely:
"Von außen sieht alles normal aus." (seperated verb)
"Von außen muss alles normal aussehen." (verb gets pushed at the end by modal verb)
It's not exactly easy, because there's no way to recognize one as long as it is not seperated. If you encounter it seperated though, you can identify it by the preposition at the very end. Usually, prepositions are followed by an object. If they show up at the very end of a sentence, they must belong to the verb instead.
Why does Duo not accept "Everything from the outside looks normal"? I can't tell you how many times I have seen examples in Duo marks wrong for perfectly correct answers. The sentence means the same thing as the translation with the same words, just a different order. How frustrating!