You’ve managed to get almost there… A bit difficult to tell from your pic, but you may have put the scissor mechanism upside down… If so, then that’s just a minor error and no worries - as long as the little pins on the scissor mechanism are intact.
(Please note that all right and left directions are based on the orientation shown in the pictures.)
Here’s how the little hooklike tabs look without the scissor mechanism. Notice that the hooks are deeper on the righthand side in the picture.
This pic shows the correct orientation of the scissor mechanism.
This pic shows also the underside of the Oops! keycap. Notice that the righthand side plastic clips are also hooklike.
In this pic the scissor mechanism is hooked to the righthand side hooks and the lefthand side isn’t yet in correct position.
It is advisable to push the scissor mechanism towards the left before trying to hook the pins under the lefthand side hooks.
In this pic you’ll see the scissor mechanism attached correctly. The little pin is just visible under the lefthand side hook.
Now it’s relatively easy to insert the keycap in its place. It is preferable to try to hook the righthand side scissor mechanism pins under the keycap hooks. I.e. insert the keycap from left to right. And when the righthand side pins are hooked up, then just press and snap the keycap lefthand side into the scissor mechanism pins.
Or just simply snap the keycap in its place. Never mind trying to hook the righthand side pins. Has worked in the past for me, but there is a possibility of snapping a pin or two off.
Hope this helps.
If you get stuck (or something is broken), then I can send you an Oops! keycap and scissor mechanism (from a dismantled prototype keyboard).
Just send me a (private) message with your name and address (click on your icon and then click on the envelope).