I get this error when I am updating a record within a subform of a main form... any thoughts?

A field name has bee mispelled. Access thinks that its a function when you think that its in the recordsource.

Is there any reason why this would work on one person's computer but error on another's?

Here's a thought:

On the two computers, bring up Access. Then get to the Module page. Open any existing module for editing. (But don't actually edit it.)

In the menu bar, select path:

Tools>>References...

The drop-down tells you which subroutines and symbols are available to you from Office and other add-ins. If you find differences between the checked boxes on the two systems, that's one place that could account for the differences.

The other thing to consider has to do with the add-ins originally selected when Access was most recently installed. If you are doing something defined in one of the add-in libraries, that would also do it.

It also occurred to me that your subform doesn;t get the key from the form and can't tumble correctly. Maybe you have to build a dummy record with the key and then add you data. You also have to have the capability of automatically deleting this dummy record in the event that you really don't want it.