minecraft.tag.missing
The diagnostics tool considers whether or not a tag has been given to the entity. Thus, it's looking for a tag "definition", a place where the tag has been assigned to any type of entity.
Examples
# Proper tag definition
tag @e add tagIf a tag already exists on an existing entity in the world that is never specified in code. or through commands blocks you can use the .mcdefinitions at the root of your workspace:
tag=IAmATag