Before you will be able to create a Rules Action like "Set a data value" for your field, you have to make sure to 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.
And to validate if your rule is working, just add a "Set a message" (within System) as your very first Rules Action, to display the actual value (using an appropriate token for your field), after your executed your rule (and re-execute it). If that doesn't work (or help), use Rules debugging, as detailed in "How to debug a rule that doesn't want to fire?".