Is the variable defined? Your $font-weight-bold needs to be defined before it can be used.
Also, I don’t think you need to import the Bootstrap SCSS file, unless you are just wanting to have access to it for building of course since we cannot always see the “full” CSS and SCSS directly in BSS I’m pretty sure they are already included? Someone correct me if I’m wrong here as I don’t do a lot with it yet.
Is think that bootstrap is already compiled and therefore you don’t have the compiletime variables available in your scss files. This may also be the reason why you can’t easily change the themes.
That what are you trying to do is incredible bad practice, don’t do that. Assigning existing variable to a new variable doesn’t make any kind of sense at all.