Mojave update issue

So I restored to iMac to Mojave as totally unimpressed with Catalina and the backward step with the Music app compared to iTunes. However, I need to update Mojave to 14.6.2 as iMovie no longer works, but every time I try and update I get an error part way through as per image below. I then read I need to do a security update for Mojave, so I downloaded it and then get a message telling me that the 380gb I have on my main drive isn’t sufficient volume for the update!! What is going on here? Tried multiple restarts and just getting so frustrated now as only want to use iMovie and manage my library on iTunes.