Looker studio has a hard-coded limit of 1 million rows: https://support.google.com/area120-tables/answer/10145198?hl=en

The Matomo Connector for Looker Studio is limited primarily by how long Looker Studio allows it to run. If it takes too long to fetch all the data for a report, it will fail. The specific number of rows this will fail at, however, depends on the report itself.

Previous FAQ: Set up the Matomo Connector template report in Looker Studio