I have read the interesting discussions e.g. in the comments here and concluded that there should be a glide between the wovels, rather than a glottal stop, since the ie occurs within a single root, as opposed to, for example, the ii in sciigo. Yet I have heard a clear /st͡siʔ'ent͡so/ on Duolingo. I suppose there is some leeway, that's alright. But trying my best at what was meant to be the canonical pronunciation makes me feel better than "it's OK if you keep saying either way". What would the books say about this word?
There is a small amount of lee way in Esperanto because the language should be universal. Esperanto does not have a glottal stop.
Each letter of the Esperanto alphabet has a fixed pronunciation, very little influenced by adjacent letters.
I struggle a bit with these types of words, so I say sit at the beginning but I try to soften the i sound as much as possible so that it flows nicely.
Scias --> sit-see-ass
Scienco --> sit-see-en-tso
In terms of two vowels beside each other, they should both make a distinct sound. Every vowel represents a syllable. No glottal stop is required. Just like you would say sa-lu-ton, you would also say sci-en-co
S-ci-en-co pronounce it how it's written. I hear so many people say it wrong and it's noticeable.