See Prevent ESD Damage.
The card is correctly seated only when the tab on the top of the card (indicated in yellow in panel 1) is securely seated inside the receptacle on the carrier (see panel 2).
Caution - Avoid twisting, tilting, or pushing unevenly on the PCIe card as you insert it into the carrier, which could damage the carrier slot or components on the PCIe card carrier circuit board. The card's connector must be fully seated in the carrier's slot before you attempt to close the top cover.
The green latch should click into place. If the top is difficult to close, verify that the notch of the card bracket or filler panel fits around the guide post.