I promise you it can be solved by the methods you've hopefully figured out, i.e. click on a purple square, and then click on a number in a red box to put it in the square. (It has to be a number in a red box; clicking on a number in a purple-background box doesn't work, of course.)
If you've played around with all the numbers below the equation and couldn't reach 14, that's normal. But...!