Getting an error: validation failed for the field - tax exempt

Hi,

in OM form, opening the configurator from the model line.

selecting a line that should be added to the configuration, clicking on finish.

the OM form raise the following error: validation failed for the field - tax exempt
and the line is not added to the configuration.

where can I check this?

there is no value in tax exempt number in any of the lines and this item is working fine with other customers orders.

many thanks,

Adi