Screencastify is a Google Chrome extension (typically). If LockDown Browser is detecting that Screencastify is running, the LockDown Browser session will not be permitted to open until you disable or uninstall it.
1) We recommend that you first open Google Chrome, and remove the Screencastify extension completely by going to Settings More Tools Extensions.
2) In addition, you may also need to remove any user data that is cached here: C:\Users\userid\AppData\Local\Google\Chrome\User Data\Default
3) You may also need to remove Google Chrome, delete the entire C:\Users\userid\AppData\Local\Google\ folder, then re-install Chrome.
4) Some students have found it necessary to also delete Screencastify from all of their Google accounts in order to get things working.
If you are unable to remove this extension, you will need to obtain another computer to take your LockDown Browser required exams. The software developer provides these instructions, which may also be helpful: https://learn.screencastify.com/hc/en-us/articles/360049985473-How-do-I-uninstall-Screencastify-