Make a Custom Field required

Field level requirements

                                           

This is the most restrictive of requirements, and it requires the field to be entered all the time, regardless of how the record is saved (i.e. through an integration, the API, mass upload, or through the User Interface). 

On the page layout

This option only makes the field required when the specific page layout that you set this requirement on is accessed. Therefore, you could technically make this required for some users that use a particular page layout but not others. 

NOTE:  This requirement only applies when the record is edited on the User Interface.

Validation rule requirement

The simplest validation rule to make a custom field required looks like this:

ISBLANK(CUSTOM FIELD API NAME)

Or, if the field is a Number or Currency type field use this syntax:

ISNULL(CUSTOM FIELD API NAME)