Overview
I see that a lot of players have a common bug that their mouse cannot click on Skirmish for whatever reason. This guide will help you to solve that.
Solution 1
Go to settings, click on resolution, change it to a random resolution but remember your original, Once you changed your resolution you should be able to click on Skirmish and you can change your resolution back to what you had it
You will have to do this everytime you start the game and you want to click on Skirmish
Solution 2
This solution is a little it more difficult than the first one.
Put your mouse next to the skirmish button. Put your mouse where it is off the menu screen but still next to Skirmish. Your mouse should now be on the rock. Slowly move your mouse about 2 centimeters down then very slowly move your mouse towards skirmish. If done correctly your mouse should be blue and skirmish is selected while your mouse is not even on the menu.