Diagnosis
You want to go to the formula editor for a particular formula.
The formula you selected does not, however, belong to the "arithmeticformula" category. You can only use the formula editor if the formulain question belongs to the "arithmetic formula" category.
For other formula types (for example, value from the previous period),necessary information is maintained in the detail screen for therelevant formula.

Procedure
Check the formula category for your formula for the key figure target.