The Mac Pro 2013 like other Mac models of that era uses a PCIe SSD with a proprietary (Apple) version of the standard M2 connector. This means a standard M2 SSD will not fit. This in turn means that if you ask Apple they will probably tell you you need to get a replacement part from Apple.
OWC do make an SSD with the same proprietary Apple connector meaning it will fit with no extra effort.
However there is now on the market an adapter to allow you to fit standard M2 SSDs in to Apple's proprietary socket. See - https://www.amazon.com/Sintech-Adapter-Upgrade-2013-2016-2013-2015/dp/B07FYY3H5F/
Furthermore whilst originally the Mac Pro 2013 only supported AHCI style SSD drives Apple included in a firmware upgrade the ability to also use NVMe style SSD drives. This is important because the older AHCI SSD drives are becoming rarer as all current computers - including Macs now use NVMe SSD drives. You would first need to ensure your Mac has the correct firmware before trying to fit one of these SSD cards. This is done by first installing Mojave on a current internal or external drive and during the Mojave installation it will if needed update the firmware as well.
You could then buy a new standard NVMe M2 SSD and use the above Sintech adapter to let it be fitted in to Apple's proprietary socket.
Note: Do not get a Samsung Evo Pro 970 as this is currently causing problems with the Mac. Instead get a Western Digital Black SN750.