A family of Microsoft relational database management systems designed for ease of use.
John (NPI),
Don't know, but you really don't give much information to go on.
Why type of Project 2016 are you using (e.g. stand-alone, Project Online, Project Server environment, etc.)? Also, is it fully updated (subscriptions forms of Project are automatically updated)?
What view are you using? Is the Custom Fields icon greyed out for all views?
If you open a new blank Project file with no tasks, is the icon still greyed out?
John