Forms won't let you use the same variable name for multiple fields.  So if there is a field with that variable already on the form you can't reuse that variable name, you'll need to create a new variable name.

It is a new variable.... but it is weird that the other fields use the same variable for all the other forms, but in this case I'm not able to do that with this new field.

Hi Erick,

This sounds like an intermittent issue. I would suggest opening a support case with your VAR and provide product version and problematic business process for further investigation.

What version of Forms are you using? There was a version that I recall this happening with where after you added a new variable to a form and then went to a different page in the same Forms process that variable was not available. You would have to go back to the form you created the variable on and refresh the page before it showed up anywhere else. It was fixed in later versions though.