Hello, My name is ***** ***** I'm going to help you today.
So the first thing you're going to need to do is adjust your clutch.
You can do this at the lever or down on the engine case where the bracket holds the cable.
The reason you can't shift is because you're not disengaging the clutch enough so there's tension in the transmission from the clutch basket.
By adjusting the clutch out you'll remove the slack in the lever and when you pull the lever it will fully disengage the clutch allowing you to shift easier.
If you're engine was low on oil there could be other damage that an adjustment won't fix. So keep that in mind if adjusting it doesn't help.
By running low on oil the internal temperatures of the engine are higher, the friction on parts is more extreme and you could have damaged the clutch to the point where you can't adjust it correctly and may need to have it replaced.
For now lets hope it just needs adjusted and that solves it.
What you want to do is turn the cable end adjusters so there is just a tiny bit of lever slack left. So you will be able to pull the lever about 1/8 inch before you feel any tension from the cable.
Give that a shot and let me know if you have any further questions.