Please check for the following cases.

• If the user is with a trial license, user will be restricted in uploading more videos. In this case, we recommend assigning M365 Stream license and retry the migration.
• If in case the configured account does not have a Stream admin access, then also we will have issues in migrating the videos. Login with a user with Stream admin access and retry the migration.
• One user account can post 5,000 videos in a particular tenant. If they cross this limit they cannot post the further videos to the same tenant. So we recommend to migrate the videos to another tenant.