PP crashes after this message, "The preset used by one or more sequences in this project..."

So when I start up a particular project I get this message... "The preset used by one or more sequences in this project requires third party components that could not be located. These sequences will be modified to use a custom sequence setting instead. To continue editing using the original preset, quit the application without saving the project, reinstall any third-party compnents that are required to open the project."

Then when the project finally opens I find that many clips in the timeline are brown and now offline. After 1-2 minutes at the longest the project crashes. 

I don't know how to rectfy this but it causes stress and the thing of sleepless nights. Can anyone help me ge past and solve this problem?

I use Adobe PP 2018.
A few months ago I did an upgrade on my 2016 Macbook Pro from El Capitan to Big Sur.

I don't believe I moved any project files on my external hard drive to a new location either.
I think the offline clips are old SD .dv clips, but it's hard to say as I can't see the brown clips in their timeline for long enough before the crash.

(Let me know if anyone needs more information from me and I'll do a re-edit to this start post)