Can I install a GTX 1050ti Low Profile in Dell Optiplex 990 SFF?

I did a quick search on google and it technically should work as long as it doesn't require an external power connector.  While most cards require a minimum of 300w PSU, the graphics card will only draw 75w max from the PCIe bus.  You will have to also factor in that you have a PCIe 2.0 x16 slot and the card is a PCIe 3.0, so while it is backwards compatible you will not get full performance from the card. Also there have been several reported issues with some graphics cards requiring UEFI bios and not being compatible or being glitchy with standard BIOS>

Optiplex 990 uses    Intel® Q67 Express Chipset. UEFI is NOT required for Pascal Cards.

PCI-E 3.0 works fine with many 2.0 chipsets.

Its the 1.0 chipsets and bios that have issue Like the Precision 490 and 690.

1080TI pascal card has been tested all the way back to XPS400 non UEFI 2006 system.