A table is not honored as source data in ArcGIS Dashboards.

Workaround

Add the table via Arcade expression:

 // table from map service, sub-layer id=2 (the table layer)

var portal = Portal('https:/PortalURL/portal');

var Tabelle = FeatureSetByPortalItem(portal, 'ItemID of the Map Services', '2');

Tabelle

Steps to Reproduce