Conditionally Locking A Range

Here is an example formula for your data validation that will only allow a selection if F1 and F2 are both Yes.

Select C6:C10 and click on Data Validation and enter this formula in place of the existing formula.

Formula:

=IF(AND($F$1="yes",$F$2="yes"),$L$5:$L$10)

This is for your column C. Repeat for column D making the necessary changes for the range.