At last moment you got a requirement to disable a particular field on a form. As you tried to navigate to access forms personalization what you see is
Navigate to System Administrator -> Profile -> System
There is no 'Diagnostics' option to navigate to Help -> Diagnostics.
Well the Diagnostics menu entry is controlled by profile option "Hide Diagnostics menu entry"
Query for your user / responsibility for which you want to provide access.
Set the value at respective level to
NO = If you want to show Diagnostics menu
YES = If you want to hide Diagnostics menu
Change the responsibility to take the profile option change into effect and try to access diagnostics menu.
Now we are able to see the diagnostics entry.