I have a number of queries written in BigQuery that work fine on their own and have variables attached (all are Field Filters) - see screenshot.
When I add those queries to a dashboard and add a dashboard filter, I can't connect the filter to the individual questions as it claims I need to create a variable in those questions first, despite them existing already.
You can see the filters in the first screenshot above ('External ID' and 'Company name'), and I've attached where I've set it up in the Variables tab too!