pump
The pump may have sawdust blocking the inlet. The pump must be pulled.
This is quite easy. Remove the black handle frame. Once off, turn the saw over. On the bottom you will see a plastic fitting between the oil hose an the pump inlet. Take out 1 screw and remove it. The pump is in the hole.
To remove the pump, you will need a 5mm by .8 screw. Used in any Stihl saw with a metal case. Measure the depth of the pump in the hole. The pump must be re-installed at the same depth. Screw the metes screw into the pump and pull it out of the hole. You may need some needle nose pliers to help pull it out, as it should be pretty snug.
Look at the slot on the side of the pump. It should be clean. If not, clean it with air or a spray cleaner. Put a drop of oil on the hole, it should spin freely.
Also remove the pickup hose from the tank. Clean it and check for cracks and leaks.
To re-install, look at the pictogram next to the hole in the case. It will show the correct orientation of the pump in the hole. Install the pump to the same depth. Reassemble the saw, using a LITTLE sealant around the plastic elbow when installing.
If this does not cure the problem, you will need a new pump.