Have you never heard the saying "To err is human; to forgive, divine."? The word "err" sounds old fashioned but is still used in certain common constructions like "It is better to err on the side of caution".
The sentence you quote and other possible translations like "Do you think he'll err" sound stilted, but are correct (and possibly better than Duolingo's "Do you think he will miss it?" which to me sounds like someone asking whether he will suffer as the result of a loss of some item or another, or will not make it in time to catch a bus or a train) rather than "Do you think he'll make a mistake" which is probably what was intended.
The verb "errar" is both transitive and intransitive. It is used without an object in the Portuguese sentence so I just wondered whether simply "Do you think he will miss?" keeps the meaning a little better than "... miss it?". For me, as I explained in an earlier comment, the version with "... miss it?" is open to several unintended interpretations.
Without the object, "miss" would mean "not hit the target." It could only be a comment by someone speculating on someone else's future attempt at shooting or throwing or swinging at a pitch in baseball. If it can mean that in the Portuguese, then it's a very fine translation. In a different context, however, the rendering would have to be different.
Yes it is. He will make a mistake > Ele cometerá um erro. He will screw up > Ele errará. (But if you mean 'he'll screw up the whole thing', it'd be 'ele estragará tudo'.) He'll get it wrong > Ele errará isso. (But if you mean 'he'll misunderstand it', it'd be 'ele entenderá isso errado'.) He will be wrong > Ele estará errado. He is in the wrong (meaning morally incorrect) > Ele está errado.
Do not think about ele/ela as corresponding to he/she. Just remember they are third person singular pronouns. Then use the general rule: Words ending in -a are almost always feminine and words ending in -o or in a consonant are almost always masculine. I tend to accept that words ending in -e are masculine, but there are many exceptions, so you have to remember each.
I'm not sure if I understood your question, but, usually, to get confused = (se) confundir:
Do you think he will get confused? = Você acha que ele vai se confundir?
Oops, I got confused = Ops, eu me confundi
I think you guys got confused = Eu acho que vocês se confundiram
Does that help? C: