Installing m.2 2280 Into slot next to battery ... slot appears to be too short

I just purchased this Precision 7740 and wanted to add  NVMe Storage. I have an m,2 2280, opened up the battery case., uninstalled the battery, and tried to place the m.2 in the slot, which has a blue slider "something" at the bottom of the slot. I thought I'd place the back end of the m.2 underneath and slide it down until I could slide it forward into the connection slot. It will not go back far enough and I cannot connect the m.2. 

I assume I am doing something wrong. Can somebody help? Thanks