Welcome to the community!
Are you able to send regular texts or SMS?
Have you verified your phone is using the correct APN? Since the phone is LTE-capable, it would need to use ltemobile.apn. You can verify the correct settings here.
Have you enabled data? Multimedia messages (MMS) including pictures and video require data to be enabled. You do not require a data plan to send/receive MMS. If you do have a data plan, the data within the MMS does not count against any data allotments. Note, however, the data within MMS will be considered roaming data if travelling outside of Canada.
Hope this helps
Did that help? Are you able to send video messages now? Let us know if and how it's been solved so we can help others that might have this same question