Why does this window pop up?

[Marq]

I have a form I plug info into which feeds a table. Sometimes when I go to plug in the PO# (highlighted in yellow, red circle around it) this window pops up (circled in red)....why does this happen? How do I fix it?

When it pops up, I "x" out of it then continue on with populating everything else with no issues.

 

[welshgasman]

The source for the combo expects a job number, and it has not been supplied (or as generally happens, has been spelt incorrectly).

HTH