No, you don't have to use the recommended needle, but you should try to be close to the recommended needle on the label of your yarn.
I've never heard of a size 14 needle. The US sizing jumps from US13 to a US15.
Are you using a 14mm needle? Is that it?
Regardless, if you use your large needle with this yarn, your outcome could be holey and loose. A
nd, the scarf could end up being a mile too wide.
Holey and loose can be a nice look for a scarf, don't get me wrong. I've knit certain scarves with a US17 using a yarn that says US9...but these scarves are garter stitch, and the outcome is what I desired. Holey and loose!
If you're using a pattern for this scarf, with a certain yarn, try to use a needle that's smaller IF you want your scarf to turn out like the picture of it.