The auto sandbox in the free version is a rather limited sandbox.

It will offer an option to recommend some files to be sandboxed (depending on various checks). It isn’t a fully fledged sandbox as such so you wont really have much control over the contents of the sandbox.

If something generates an autosandbox prompt, it is not an indication that something is necessarily wrong, but that avast feels that it could potentially be bad. A better be safe than sorry approach, if that makes sense.

The settings that are available for the sandbox can be found here:
Open avast → Additional Protection → Auto-Sandbox → Settings