Not able to select a model or question to supply values to Variable in custom SQL question

Earlier when I used SQL as a saved question to supply values to a Dropdown text filter it worked perfectly.
Now using the same process I am not able to get the list of the values that my query returns instead I'm getting the error "An error occurred in your query"
My query is simple SELECT some_col FROM my_table.

The column is in STRING.

Attaching the screenshot where I'm stuck currently.