According to Dell's Specifications, this is all your laptop supports:   So, you may use any 2.5" SATA SSD, but not an NVMe as there is no NVMe slot. Here are some compatible Crucial SSDs, but you may use any brand. I have put the Crucial MX500 SSDs in older Dell laptops and they work well, with good performance at a reasonable price. You may choose any capacity you want, all the way up to 4 TB. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
Hello, I am looking for some help and advice and am hoping someone may be able to help. I have an Inspiron 15 5568 2-in-1 and was wondering whether it is possible to fit an NMVe SSD drive to it.  I have already fitted a SATA SSD, but would like to add a second disk if possible.  It looks like it has the data port on the motherboard, but perhaps not the mounting bracket? Can anyone advise? Thank you in advance.    
NVMe for Inspiron 5568 2-in-1