"We are going to be with you."
Translation:Εμείς πρόκειται να είμαστε μαζί σου.
That's grammatically incorrect -- your να is not followed by a verb, and είμαστε followed by another finate verb doesn't work either.
It's like trying to turn "I used to take the train when I was small" to "I take used to the train when I was small".
You have to have the order "used + to + take" or "πρόκειται + να + είμαστε" for it to make sense.
Perhaps you're trying to think of the να as "belonging" to the πρόκειται? It's perhaps best to think of it as belonging to the entire construction.
"Εμείς" doesn't need to be included it's perfectly fine without it. So, if you wrote this sentence without "Εμείς" it would be correct and accepted.
However, you may have had a mistake with something else.
You should first check the correct answer shown to you then if you are sure your sentence was correct you should have made a report.
Here's how to REPORT.
Go to the bottom of the exercise page where you'll see "REPORT"
Click on that:
Then choose what you want to REPORT:
-My answer should be accepted. This is the one you should click on if your translation was not accepted.
After choosing the REPORT you want to make you should post on the comment page.