The studyplan plugin will automatically add links to the flat navigation bat used in Moodle 3.11 and below
However, Moodle 4.0-4.2 use a new "primary navigation" bar, and does not yet support customizing the primary navigation bar (it is a planned feature though)
**Use the following workaround:**
You can add custom primary menu items in **Site administration** -> **Appearance** -> **Theme settings**
That page contains an item called "Custom menu items"
Add the following into that area to custimize it
// [your name for my studyplan]|/local/treestudyplan/myreport.php
// [your name for studyplan viewing]|/local/treestudyplan/view-plan.php
// [your name for studyplan managing]|/local/treestudyplan/edit-plan.php