Currently it is not supported to link a call, e.g. created from the Account Detail Page or in the calendar to an Account Plan. This link is only available when planning directly from the Account Plan.
This behavior was supported in the past but has been changed in one of the last releases. Veeva support told us the old option to create the link for existing calls was seen as a bug.
We see so many users complaining that they now have less flexibility in their planning process and therefore really demand to have this option available again.