You can't get cell values as Pivot Table values - they must be either Sum, Count, Average, Max, Min, Product, Numbers count, StdDev, StdDevp, Var or Varp of COLUMNS values.

As Pivot Table is 2-dimensional as max, with 3 fields you have to decide, do ROWS or COLUMNS have 2 fields to group. And as values you have to use Count - there is now way you can sum letters, and I can't see a point in summing F1 values too.

You can get the result more like you want, when you add a calculated column where you concatenate values in F1 and F2