This is how we initially configured a list lookup we left the ID connected to set to its default setting.
In new development we noticed that the lookup columns were no longer displaying the user’s selection.
We found guidance that indicated a non-default value must be selected for the configuration column ID connected to. Figure 2 below:
However, when we selected the lookup list column Vendor for connected to, the Selected List column reverted to its default value –Please Select – and triggered the warning message in yellow the Source List is required. When the connected to column is reset to Not connected we can see that the Source List configuration field is set to its default value. See below:
How can I configure the lookup control to save the user’s selected item and make it visible in the item’s row?
I had the same problem.
I did delete the List Lookup from the custom list and then re-created a new one.
Then went to nintex Form and the ID Connect To fiels does not show the error as you have explained.