What worked for me was saying the word "practice" with a D instead of an R so its like "pdactice" , try saying it faster and faster and you should find yourself rolling the "r" as you transition from the P to the D. Sounds odd but i can now roll Rs after doing this, hope it helps :)
Try making a fluttering sound by blowing air across the top of your tongue while keeping the forward half of your tongue muscle relaxed. Think of how you might imitate a cats purring. Once you have that at least partially mastered then try to pronounce an "R" sound at the same time. Hope this helps you out in learning how to roll those "R" sounds. :)
Again context is everything. There are professional "taste testers" who test food samples for flavor quality by testing the taste of various samples. At the "Panda Express" cafeteria the servers ask, "Would you like to try a taste?" If you say yes they stick a toothpick in the type of food you indicate and after tasting you decide if that is what you want.
"Pruebar" must be a Spain Spanish kind of word. Living in California nearly my whole life, it's the first time I've encountered this word. I've also noticed the auto-speech voice doesn't sound very American Spanish. Sounds much more Barcelona-like. To be expected, I guess. After all, that is a Spanish flag, not Mexican or Argentinian.