Being that the Source List drop down seems to be populated, I would make sure to see what the actual internal name of the column you're trying to reference in the List Column Name is. 

Here I have a Lookup Control referencing an out of site list:

I want to get the Product Styles column from the list, as we can see in the List's settings:

But let me check the actual internal name by *clicking* on that column name:

Just in the edit dialog, the Column Name looks to be exactly what it says:

However, the URL bar at the top of my browser tells me the real story. It's internal name is actually "Title":

So instead of using the external name in my Lookup Control, I'll use the Internal Name:

Further Information

This can all be a bit confusing because sometimes both the Lookup Control and Lookup() Runtime Function will let you get away with using the non-internal name, but that's because it will attempt the query using what you typed first, and if it fails, attempt the query again after converting the column names into the formatting of how they would be as a SharePoint internal column name.

If I were to look at a column like "Product Category" that I made instead of renamed, I can see that the internal name is Product_x0020_Category:

So if I had looked up using the name "Product Category", it would fail once, then attempt it again using the converted name "Product_x0020_Category" which just happens to also be the internal name.

However if I created a column that originally called "Name" and then changed it's externally facing value to "Name1", and then used that value "Name1" as the column name to return in my lookup, it might fail, then try to convert it, but because there are no special characters to convert, it just uses the same value "Name1" again (which fails - again). 

The same thing would happen had I tried to use "Product Style" instead of "Title". The lookup would try to:

1. Make a query using "Product Style"
2. Fail because no "Product Style" internal column exists
3. Make another query using "Product_x0020_Style"
4. Fail because no "Product_x0020_Style" internal column exists

For more information about how SharePoint Internal Column Names and the conversions used for special characters, see: https://abstractspaces.wordpress.com/2008/05/07/sharepoint-column-names-internal-name-mappings-for-non-alphabet/

Please let me know if that works or not!

Thank you for this but I'm still having the same issue, blank.  

I tried connecting to other lists on that site and continue to have the same problem. I have Full Control of all lists on the site I'm referencing. I've tried ID and Title and continue to return blank "Please select a value..." Thanks again for your effort on this!

Could this be an issue with my site settings? Everything labeled Nintex is turned on. 

No Problem, we can keep digging!

What do your filter options look like? 

Other things

Here's another fun thing you can try which might be informative. Go to SharePoint and press the F12 button. A Developer Console from your Browser should open up (there might be a notification asking if that's what you really want to do depending on the browser you use). It may already be on the Console Tab, but if it isn't, find it and click on it:

(note: your dev console may be a different color if you're not using the dark theme everywhere like I do :P)

In SharePoint navigate to your Form Edit Environment:

And then fire up a Preview:

In the Dev Console's Console, do you see it populate with any information or errors?

If so, drop em here!

Thanks for all the help! I pressed F12 in Edge and received this message. I continued to receive the same message every time I reloaded the page:

I tried it in Chrome and nothing popped in the main window. I did however receive three issues:

Thanks again!

As far as filtering for my List Lookup, here's what I have:

Those dev console issues aren't anything particularly special and I think are just due to how sharepoint might be doing things. As for your filters answer... well that rules out that! 

At this point I would make sure that the query works at all regularly, and utilize the Query List action in the Workflow Editor. 

Configure the action for your SharePoint environment:

a basic CAML Query based on the info you've provided thusfar (it's just going to get all of the items in the list, but only return the Name1 column value of each item returned):

Then click on the Run Now button:

When you click on the Execute button in the resulting window, do you see any results?

Sorry for this but I have no idea how to get to where you are showing me. 

Just using the Query List Action in the Workflow Editor.

Do you only have a License for Nintex Forms?

If that's not the case, and you do have Nintex Workflows as well, you can simply go to a list and create a new workflow:

You can then just search for Query list and drag and drop the action into the editor: