Enter the reverse of the first entry that had no prices.   This is needed to adjust the inventory quantities back to what they were - i.e. if you recorded that you purchased 9 widgets for nothing, record that you returned 9 widgets for nothing.

Enter the invoice again, put a period after the invoice number if Sage 50 complains that it is a duplicate.

When no prices are entered on a purchase invoice, no G/L accounting entry is created and there's nothing for the 'adjustment' to 'hook on to' to open it. 

Thank you. I did that however both invoices ((one with period and one without (original)) still show up in Simply. How do I delete them from the SA?

They're part of the permanent audit trail, only not flagged as 'reversed' or 'reversal' transactions like automatic adjustments are. The only way to completely remove mistakes is to restore from backup, and then not make them. I usually just make different ones.