I struggled with this as well. Firstly, ensure you've followed the "Autocompletion" Setup described on the website; for example, they require something like the following exist in a file called jsconfig.json at the root of your project:
{
"include": [
"**/*.js",
"**/*.json",
"node_modules/@wdio/sync",
"node_modules/@wdio/mocha-framework"
]
}
You may already have a jsconfig.json; if so, ensure that the node_modules directories are not in a section called "exclude": { ... }. When setting up other things like Babel (for mocha) this may get installed as a default configuration entry. When node_modules is in both include and exclude, exclude takes precedence.
I had zero success importing npm @types pacakges, adding typeAcquisition: {} to jsconfig.json, or adding interface browser; into the file global.d.ts as other people have suggested in various forums.