Did the standoff come with the case, or with the motherboard? There are typically 4 kinds of screws in a desktop computer. Thumbscrews can, but shouldn't be used to fasten motherboards, as they can be awkward and the heads can be too large. Fan screws are wrong. M3 hex screws might work but they will not hold well. I suspect that's what you tried. In most instances, the right screw for motherboard mounting is the #6-32.

The right screws usually come with the case or the board. Any computer shop will have handfulls to give away.

Going without standoffs is risky, as you may short out the board.