9020 MFF motherboard slot 1 M.2 slot is for wireless LAN.  spec does not mention m.2 ssd.

9020M motherboard does have a slot 2 M.2.

PS 3020M board does not have slot 2 M.2 soldered because it is a cheaper entry level model.  there is no 7020M or 5020M.

OK if that's what you're saying why is there some people say that the Dell Optiplex 9020M can support a m.2 ssd . Can you please explain that to me 

Take a look here all I'm asking what m.2 generation the dell Optiplex 9020m support 

9020M motherboard has slot 2 M.2 for ssd such as this one (Y5DDC)

the chipset is Q87 PCI Express Revision2.0

https://www.youtube.com/watch?v=vYdFP6OHSO8

Now since you found the motherboard what generation of m.2 this take what m.2 gen? That's my question to you

gen 2.0 because that is what the chipset has. gen 3 ssd is backward compatible and will run at gen 2 speed.

9020M board slot 1_M.2 is socket 1, for wireless LAN.

9020M board slot 2_M.2 is socket 2, for ssd (sata or PCIe x2).

M.2 connectors on the host are called "sockets." Each socket has a unique mechanical key. According to PCI Express M.2 Specifications Rev. 1.1, the sockets are distinguished as follows:

• Socket 1: Connectivity socket for Wi-Fi, Bluetooth.
• Socket 2: WWAN/SSD.  the socket may accommodate a B+M-keyed small M.2 SSD.  Socket 2 configuration is for SATA or PCIe x2
  
  
• Socket 3 configuration is for PCIe in a x4 configuration