You obtain a patent which is a new use for an existing patented invention. The patent of the existing patent is still in force and valid. If you want to use your patent, do you have to get a license from the owner of the of the existing patented invention?
If your invention would be infringing on that other patent, then yes, you would have to obtain the rights or license from the patent owner to build or sell your invention.
All a patent does is give you the rights to "exclude others from making, using, offering for sale, selling or importing [your] invention." But a patent does not guarantee you the right to "make, use, offer for sale, sell or import [your invention]." — What Are Patents, Trademarks, Servicemarks, and Copyrights?
So even if your patent is completely valid, it does not grant you the unencumbered right to build and sell your invention. It only gives you exclusivity to keep others from building or selling it.