There is no formula for sick time used in column G. You have to enter the time as it is used.
Sick time available can be calculated as the difference between the time accrued and the total amount previously taken. In H4, enter the formula =F4-SUM(G$4:G4). Then copy this cell to H5 through the bottom of your data. This will cause the reference to F4 to access the current row's data and the sum function to calculate the total number hours used to date. For example, cell H5 will contain =F5-SUM(G$4:G5).