Are you using the lookup, or the Adjust button?   Adjust will only allow you to open invoices back to the date set under Setup | Settings | Company | System, as the earliest allowed transaction date.

Unless the sales invoices have been purged ('Cleared'), they are normally accessible for several years (depending on the Edition of Sage 50)

Can you access older invoices by 'drilling down' from previous years' financial reports?

I am going under sales invoices and using "find invoices" not adjust....and yes I can access older invoices by 'drilling down' from previous reports