To use the square root function in a column formula, simply raise your expression to the 0.5 power.

You can calculated the square root in the column formula calculator, but it is not a dedicated button. The square root is the number you can square to get to original number, and from algebra:

(x^(1/a))^a=x

so to get the square root you need to calculate x^(1/2)

See below: