Microsoft Stream videos depend on Microsoft 365 permissions that live outside Knowly. If a Stream video plays for you but not for participants, it's almost certainly shared with you but not with them.
Why this happens
Knowly shows a Stream video by embedding the link you pasted, but it can't grant anyone access to it. Microsoft decides who can watch, based on how the video is shared in Microsoft 365 and SharePoint. So a video you own plays for you, while participants outside that share see nothing or a sign-in wall.
What to check in Microsoft 365
Open the video in Microsoft Stream, or in SharePoint, and check its sharing settings.
Share it with everyone who's in the learning journey — the specific people, the right group, or your whole organization, depending on your setup.
If your company blocks anonymous or organization-wide sharing, work with your Microsoft 365 admin to find a sharing level that still reaches your participants.
Test as a participant
Preview the activity as a participant, or ask one person to open it, before you rely on the video in a live journey. If it still won't play, the fix is in Microsoft 365, not in Knowly. For other playback causes, see Troubleshooting video playback in Knowly.