Looker Studio doesn’t have built in support for duration types. I think you’ll need to build a calculated field that computes the total number of minutes (or seconds? depending on how your duration field is formatted) instead if you want to use it in a metric.
This article has some sample code for that, assuming the duration is formatted in HH:MM:SS:
( CAST(REGEXP_EXTRACT(HH:MM:SS, "^(\\d{2})") AS NUMBER ) * 60 * 60 ) + ( CAST(REGEXP_EXTRACT(HH:MM:SS, ":(\\d{2}):") AS NUMBER ) * 60 ) + CAST(REGEXP_EXTRACT(HH:MM:SS, "(\\d{2})$") AS NUMBER )