One or more tables in your database are too large and need to be reduced in size. For information on assessing your disk space usage see About disk storage in Cloud Platform.
Before attempting to reduce the size of any database, it is important that you make a backup as is best practice. Please take a look at our Backing up your Drupal database documentation.
After you have backed up the database, you could use the following command to connect to your database, replacing
[site-uri] with the base URL to the website you want to connect to:
drush sqlc --uri=[site_URL]
TRUNCATE TABLE table_name;
For example, if the table you needed to truncate was cache_container.idb, you would run this command
TRUNCATE TABLE cache_container;
For more information on truncating a table, please see Managing Large Cache Render tables in Drupal 8, which outlines how to truncate the cache_render table in particular, for Drupal 8.