When setting Values or Formulas (and the sisters of the Formulas property), according to the docs we're supposed to pass an array of Any[][]: https://learn.microsoft.com/en-us/javascript/api/excel/e...
FYI: The code in my example runs successfully when ts-ignore is added:
async function run() {
await Excel.run(async (context) => {
const sheet = context.workbook.worksheets.getActiveWorksheet();
const rng = sheet.getRange("A1:A10");
//@ts-ignore
rng.formulas = "=B1";
await context.sync();
});
}