To confirm when you map the custom form estimate you have created there is an option to map terms to your field on your created form but it does not accept it? Could you attach a screenshot of what happens so we can investigate this?
I appreciate the screenshot you've provided, @PMYORKS.
The terms field is one of the customized fields when creating estimates. When creating an estimate, there's no need to add terms on it. Since this specific field refer to the agreement between you and your customers as to when the invoice payments are due.
However, should want to include the Terms in importing your customized estimate template, you can assign one of the Custom fields to add the Terms. Then, manually add the terms on it.
Know that the Community and I are always here to help if you have other questions.