@SusanFinnis ...
My understanding is that Lockdown Browser doesn't allow you to use other applications on your computer while taking a quiz/test/exam. If you take a look at this page:
LockDown Browser - Respondus
...you'll see a section called "How LockDown Browser Works".
You might want to speak with your instructor more about the issues you are having with the practice quiz. Chances are, another student in your course might be experiencing the same thing. You can reach out to your instructor by following the steps here: How do I get help with Canvas as a student?.
I hope this helps!