If there is a data entry problem on a page, the tab(s) to the right of the tab with the data entry problem will be disabled.  For example, in the image below, a data entry problem exists on the Retirement Income tab.

To determine what the data entry problem is, select the last available tab (the tab farthest right that is enabled).  There will be a "View Error Messages" button at the top-tight of the page.

A message will be displayed about the problem with the data on the current tab.