TK is an abbreviation for to come in journalism. It is used as a placeholder for a piece of information that is not yet available but is expected to be filled in before publication. Why not TC? Nobody knows, but the best explanation is that it is intended be highly noticeable so it will be caught and replaced with the proper information. Occasionally it slips through anyway and you might see it in print.

For example, a reference to another story that does is not yet assigned to a particular page might refer to page TK.

In contrast, TBA is a publishable abbreviation for to be announced which simply indicates information (especially times) not available at publication time.