As a Zoho One user, how do I rectify the error “Unable to add users. Please contact support@zohoexpense.com.”?

You need to be an admin user in both the Zoho One and Zoho Expense applications. If you’re using the Zoho One or Zoho Finance Plus bundle, you cannot add users directly to Zoho Expense. Please add users to the bundle first and then assign Zoho Expense to them.