What does the checkbox do? IOW what is the purpose of it?
it just shows if the record showing on the layout has any related records in the other table.
You should use a calculation field for this, not auto-entered value. The formula can be something like:
not IsEmpty ( Related::Matchfield )
Well, if you don't NEED a checkbox (since it can't be checked anyway) ... and since you simply want to know if there related records for this main record, you could just place the related ID on the main layout (be sure to disallow entry into this field). Even better if the related ID is a number, set the number format to boolean YES or RELATED. Then no calculation would be necessary.
One could even use conditional formatting ... setting a text label in black called RELATED and set to turn background color if no related record.