You need to download popper.js yourself. Bootstrap uses it, but it's not included in the bootstrap code.
As you can see on the homepage of the bootstrap docs they say that you need to include the CDN for popper.js yourself
I found this but of text in the docs
Bootstrap is dependent on jQuery and Popper, these are defined as peerDependencies, this means that you will have to make sure to add both of them to your
package.jsonusingnpm install --save jquery popper.js