Yes, you can drive free of charge on any non-toll roads, but you will be subject to a heavy fine as soon as you enter a tolled highway or expressway due to zero-tolerance rules.
Related articles:
Which roads require a Slovenian motorway vignette?
What happens if I drive without a Slovenian vignette?
What different types of the Slovenian motorway e-vignette are available?
How is the Slovenian vignette checked?
Can I drive until the first exit on the motorway in Slovenia without a vignette?