Why can't I open reconcile on the Mac update of Quicken 6.5.1?

If the Reconcile button at the bottom of the register (or in the menu) is gray, it's likely that you have clicked on an account group rather than a single account in the left sidebar. Account groups are the headings like Cash, Credit Card, Banking, All Transactions.

If you see only a heading like Cash with no accounts underneath it, the account group is collapsed. You can hover your mouse over the Cash heading and click on the ">" icon which appears to the left of the word Cash to expand the group. Or you can click the ••• icon at the top of the sidebar and select Expand All Sidebar Groups from the menu.