Quicken for Mac 6.5.1 update

Can you post a screenshot of the error message here? You will probably want to contact Quicken Support.

In the mean time, the previous version of the Quicken app can be found in your Trash folder if you'd like to revert to that version (probably 6.4.x). Your current data file is probably not damaged, but if it is, there is a Pre-Update backup of your file that can be found at HardDrive > Users > (youruserid) > Library > Application Support > Quicken > Backups > Automatic Backups. To access the User Library folder, hold down the Option key while clicking the Go menu in the Finder.

Before you revert to 6.4.x, you may want to attempt to restore the pre-update backup and see if 6.5.1. will convert it successfully. Maybe it was a one time glitch.

No, the forum is not monitored by support. It is primarily user-to-user with occasional contributions by the Moderators (who are not part of the support organization).