I want to buy an M.2 SSD for an Acer Aspire 5 A515-51G-390G laptop. Will the Samsung 980 fit?

This is what Acer recommends for your Aspire A515-51G laptop for its M.2 drives but the A515-51G accepts both B&M SATA-3 M.2 drives and M end key type PCIe 3 M.2 drives as some community members have fitted to this laptop also, below are the Acer official listed A515-51G specs of what Acer fits to this laptop for its M.2 drives and also its memory specs. I would suggest that you upgrade both M.2 drive and memory for best performance results. Btw. with the M.2 drives this laptop is not limited to a max of 256GB as in a UEFI type bios system its only limited by the hard drives format e.g. Legacy/MBR up to 2TB and GUID (GPT) above 2TB.

M.2 SSD

Memory

The Samsung 980 is a PCIe 4 x4 the Samsung 970 Plus is the best PCIe 3 x4 from Samsung, so use that as the PCIe 4 x4 your laptop will never ever utilize its speed and its wasting your money, even the 070 Plus is too much for your laptop as it will never ever utilize its read/write maximum speeds. the best drive for your A515-51G laptop is a mid-range M.2 PCIe 3 like either a WD SN550 or a Samsung 960 EVO that are cheap and will work great with your A515-51G.