I have that error from this code block
const username = comment.user.displayName ?? comment.user.firstName;
any help will be much appreciated
You need to use @babel-plugin-proposal-nullish-coalescing-operator to transform the Nullish coalescing operator.
Use
const username = comment.user.displayName || comment.user.firstName;
or
const username = comment.user.displayName ? comment.user.displayName :comment.user.firstName;