Before creating Active Contract you need to create Calculation, for more information use link How to create calculation.
Then the process of transferring status from Calculation to Active Contract is simple.
...
It's possible to shorten the process of creating an Active Contract by steps below. Follow the guide steps or go to How to work with approval process
Step n. 1
By clicking on Button Approve Contract, the contract number changes, and it's no longer a Calculation, but a Contract.
...
Posted Invoice Count: represents the number of posted invoices.
Cancelled Invoice Count: represents the number of cancelled invoices.
Invoice Count: represents the number of created invoices, but not yet posted invoices
...
Note: In case there are all payments invoiced in payment calendar and user clicks on button Calculation, payment calendar cannot be recalculated. Instead, there is displayed message: “There must exist at least one line where Rel<>0.” In case of interest model: “Number of payments not invoiced is lower than 1.”
Follow up:
How to create calculation in general