If you established your connection using an Advisor's credentials, your Lightyear permissions would be piggybacking off that user's permissions in Xero.
In Xero, when you go to lock down a period, there are two options:
This means you have rights as an Advisor when exporting bills from Lightyear, thus allowing you to export if you only have the first option set up.
You can either establish the connection using a different user with different permissions/rights in Xero to limit or select the second option depending on the best process for your business.
Consult your accountant or bookkeeper if you aren't sure how to proceed.