An error stating that the 'preview' engine has been deleted can occur when using RDBMS sources.
The error message will say:
The engine 'preview' has been deleted. Create a rule using a different
engine under Project Settings > Engine Routing, and then try again.
Adding an RDBMS source in Dremio Cloud requires an engine that will be used for metadata refresh. Currently, Dremio uses the engine named 'preview' for this. If the default 'preview' engine gets removed, this error can occur.
To fix this error, re-create an engine named 'preview' and make sure it is enabled. Though it can be any size engine, the default preview engine is typically size 2XS.
To save on costs, the engine's min replicas can be set to 0. However, if there are multiple sources refreshing frequently, replicas may never scale down to 0. In this case, metadata refresh settings can be adjusted to a greater number to reduce the load on the engine.
Engine routing: https://docs.dremio.com/cloud/workloads/engine-routing/