Not to contradict both prior answers, but IMO they are both missing (ignoring?) a crucial part of the question, which is "...I created an Action Set...". True, part of the solution is, pretty sure, "
Entity has field" (as in the answer from jcarballo). But, by design of a Rules Action Set, you cannot include any Rules Condition in it. Therefor, either of the following is needed to complete the answer to this question:
With the above in mind, you should be able to create a Rules Action like (e.g.) "Set a data value" for your field, if you also add a Rules condition
Entity has field (related to the field for which you want to set a value). And make sure to add that
Entity has field condition BEFORE other Rules Conditions in which you might want to refer to this field (if you have any such conditions in your rule).
That's also what is mentioned in the Rules UI, e.g. when you're adding a "data comparison" condition: somewhere it says "The data selector helps you drill down into the data available to Rules. To make entity fields appear in the data selector, you may have to use the condition 'entity has field' (or 'content is of type')".
For a video tutorial that illustrates the importance of this
Entity has field condition, refer to Data types and data selection, especially what is shown between about 13:30 and 17:30 in it.