There are a couple of standards that must be met in order for a search to be legal.
The officer should "articulate" why he feels the suspect is "nervous" beyond the norm. However, the nervousness may constitute "reasonable suspicion" in order to advance to the next level to obtain "probable cause" which would come if the K-9 gives a positive alert on the vehicle.
Another issue with this type stop is the length of time taken to get the K-9 there. If it takes an "unreasonable" amount of time...the search could be void.
Based upon your comment thus far...if marijuana were found the search would most likely be acceptable to the court.
If I had been the officer asking for consent, then was denied, I would certainly attempt to "articulate" a bit further about my observations beyond "you look nervous."
That doesn't mean the officer, in his report, didn't make further articulation for his suspicions. There is no question in my mind, after the years of experience, that nervousness is one of many indicators that something isn't "just right." When the officer observes those things which raise his suspicions, he can detain you for a reasonable amount of time until the K-9 arrives. 10-20 minutes is not unreasonable. Had it taken an hour while the K-9 officer ate lunch, shined his shoes, etc., I'd say that was unreasonable. However, if it took the K-9 an hour coming from across the county to get there...the court would consider the time reasonable provided the stopping officer can show his "reasonable suspicion."