Currently, Google Apps Script does not support ES modules. Hence the typical export/import pattern cannot be used and will fail.
The trick here is to make sure not to export any functions in your entry point code, e.g. index.ts, and to prevent any generation of export statement in the final bundle (see the custom rollup plugin in rollup.config.js.
- Add your script id to .clasp.json
- Put your code into index.ts
npm run build
npm run deploy