I think "brand new" would be stylistically more appropriate, and still carry the same meaning.
You're both correct, owlspotting and Sandybell, and Duo accepted "brand new," 10 April 2019. But, I wonder where this strange adjective phrase ("brand new") came from!
I probably comes from the English word "brand" referring to the glow of a freshly-forged item, but the etymology is not entirely clear.
It should be '... new bicycle', not 'bike'. Bike = bici.
Actually, Duo makes me angry when it does that. Another of Duo's quirks is grading off for something we've never been taught, i.e., one hundred and one...WRONG sucker.
What is wrong with "this is a completely new bycycle"?
There is nothing wrong with the statement in the context that if it was a broken, rusty contraption that somebody had rebuilt and made it look like a spanking new bike!!
Indeed, just what I was thinking. With that in mind I tried "It's a totally new bike!" and Duo accepted
Is bicycle wrong?
No, it's fine. :)
It's a bike completely new?
"It is a completely new bike" was wrong.. why