React (and I assume, in turn, the React Bootstrap library) uses the function
process.env from the Node standard library to check whether it’s in a development or a production environment.
It is assumed React applications will always be ran through some Node-based build process, so normally this is not an issue. You are not doing this, so you are going to hit issues.
To work as an ES6 module in browser, any piece of code using a Node API has to be replaced. The code hosted on Skypack has to be a version of the original code that conforms to ES6 module spec, and with those replacements for Node-specific code (or for other external APIs) made.
The versions of React/ReactDOM on Skypack have had this done afaik – it’s still difficult to build anything using them, but they’ll work. So if you are getting this error, I assume the React Bootstrap library has not. If that assumption is correct, then it will be unusable.
FYI, front end development using only ES6 modules for anything non-trivial that uses external dependencies is not currently practical. Almost the entire frontend ecosystem is based on the assumption that your code will be transpiled and bundled into a deployable unit.
Development of non-trivial React apps using only ES6 modules is effectively impossible: as things stand you are on a hiding to nothing attempting it. Preact works better, mainly because hyperscript can be used instead of JSX so there is no transpilation step required, but it’s still extremely difficult to build anything non-trivial if you aren’t writing every dependency yourself.