Is it possible to interrupt a linear stream with an ad-break?

A linear stream is a continuous stream which cannot be interrupted by ads served from an ad network or any other ad source, or else viewers would miss out significant moments of shows and content of your linear streams.

That said, there are multiple other options to play video commercials in a linear stream:

1. Use Lightcast's 24/7 Cloud scheduler where you can drag and drop video ads into the desired time-slot, or by using the filler-system for auto-filling blanks in the schedule. Please make sure your video ads are uploaded to the Media Cloud in order to use them in the scheduler. 

2. Another option is to place pre-roll ads in front of your linear stream served through a VAST feed of your ad-network. 

3. Or you can upload and assign video commercials to one of your empty VOD Channels which would then be placed as pre-roll ads in front of your linear stream.