ADF Flatten transformation - can't select unroll by

I have some strange issue creating ADF DataFlow. After setting up the source transformation, and seing the output with the data preview, i am adding a flatten transformation.

And then, I attempt to select "unroll by" field, and it seems as it tries to do something, but at the same time seems disabled. I do not see my result from source transformation selectable. And I can't manually type it. I expected to do something like

body.pagination.limit

I can see that hierarchy in the data preview from the source transformation.

I have another dataflow, where i did it earlier, and it seems that i am doing the same things with configuration. What am i missing ?

Flatten transformation only supports Array of JSON type of fields. If your filed is an object type then, it won't take it. So, make sure your source field is an array of object type.

If your source contains the required field and still it is giving the same result, then re-import the schema and check again in the flatten transformation.