The Cash Flow Planner is one of the newly added features of QuickBooks Online (QBO) that is currently on beta testing. Thus, some may not yet have this option.
Once we gather all the information needed from the final testing, we'll be working on making this feature available to all QBO versions including QBO Plus users.
Updates will automatically apply to your company file when the final launch arrives. By that time, you should see the Cash Flow menu under Sales. From there you can start managing the Cash Flow Planner (See attached sample screenshot below).
All you need to know about how Cash Flow Planner works in QBO can be found in the following articles: