Open the related document and press Alt-F11 or Hover over one of the sheet names and right click and then choose "View Code"
Select the sheet you want to hide in the Project Explorer (in this case the Project_Personnel)
In the Properties Window change the "Visible" property to "2-xlSheetVeryHidden"
Click on "Insert" tab and choose add Module.
Making sure the newly created module is selected continue with setting a password:
"Tools" menu
Select "VBAProject Properties..."
Select "Protection" tab
Check the Checkbox for "Lock project for viewing"
Enter and Confirm your password.
Click OK
Lastly, save the document as a .xlsm and restart the document