See also:
What is a field?
What is the meaning of the arrows?
A field placeholder is a field name enclosed between <% and %>. Email Parser will replace it with the actual captured text when an email is processed. For example <%From%> will be replaced with the email address of the sender of the email being processed. Or, as seen in the picture below, <%Temperature%> will be replaced by a temperature value produced in a parser.
You can add a field placeholder in an action by clicking on the field icon on the right or simply typing the field name using the syntax <%field%>
Actions use the fields from previously run items, usually parsers and email sources. You can check which items are executed first following the arrows of the left panel backwards:
The most common cause is a missing arrow. Check that there is a path of arrows going all the way back to the item that produces the field.