What is the exact model of the SSD you are trying to install? Also is it PCIe or SATA?

This one is supposed to be compatible with my laptop

Dell M.2 PCIe NVMe Gen 3x4 Class 40 2280 Solid State Drive - 2TB

We are here to assist you. Refer to the below methods and check if it helps.

Firstly, I suggest you to run the Hardware and Devices troubleshooter and check if it helps.

1. Press Windows Key, then click on Search bar.

2. Type “Troubleshooting” without the quotes and click on Troubleshooting.

3. Then click on View All on the left panel of the screen.

4. Click on Hardware and Devices.

5. Click on next and follow the instructions.

If the issues still persist, refer to the below methods.

Secondly, I would suggest you to try to check for the status of the drive in the device manager.

1. Press Windows Key + R Key.

2. Then type “devmgmt.msc” without the quotes and hit on Enter Key.

3. Expand Disk Drive. If you are able to find the driver for the external drive.

6. Then again right click on the driver and click on Properties.

7. Check the device status of the disk.

Kindly reply with the information, so that we can resolve the issue.

It didn't help, unfortunately. My computer just doesn't see it.

So, Dell's tech support person told me that what I bought, Dell M.2 PCIe NVMe Gen 3x4 Class 40 2280 Solid State Drive - 2TB, isn't compatible with my computer, even though Dell's salesperson said it was and Dell even sells it in their "compatible parts and upgrades" page for this laptop. Dell support told me that the 2TB 4x4 version is what's compatible, but mentioned that the 1TB 3x4 will work.

So I got that, and I'm having the same exact issue. I contact tech support again and they're saying that I have a defective item and suggest I get a replacement. I'm getting so tired of this. It's been a whole month, and I have gotten nowhere. 

They need to train these guys better. Pcie 3x4 and 4x4 are cross compatible. Both types of drives should work in both slots just the speeds will vary.

Check if the SSD is being detected in the bios. If it is, you know it's not a hardware issue.

To answer your question, one reason i can think of is the default windows installation is done with Raid mode for some reason. Raid driver makes 2 drives appear as one.

Check if storage is raid/vmd or ahci/nvme. If it's raid, you can backup everything switch the mode to ahci and reinstall windows. This should make the SSD appear in disk management.

If u don't wanna reinstall windows, Dell actually has a guide that you can follow here:

https://www.dell.com/support/kbdoc/en-in/000137098/windows-10-new-ssd-not-detected-in-disk-management