Errors in the GEDCOM are reported using Reports ⟶ Error Report ...

List of errors in the current GEDCOM file

Click on individual sections to see a breakdown of the errors.

GEDCOM validation, Family relationship errors
(Click on the image to enlarge)
 Viewing the relationships in the generation chart, is a good way to visualize the errors

Visualizing the relationship errors in the chart
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 The lines in the chart represent family relationships.
The error "Parent is Child" is visualized by having a family where the person is both parent and child.

GEDCOM validation, Broken links. List of broken links
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 Click    to view the original GEDCOM
 Click    to refresh the report after apply fixes.
 Click    to view the entire GEDCOM file.

Original GEDCOM, viewed in java. (this is not viewed in the browser, but appears in the toolbar).
(Click on the image to enlarge)

Errors for folks on the current chart can be seen using Right Click ⟶ View ⟶ Chart Info (q)

Viewing errors on the current generation chart

The context menu on the chart, is helpful for fixing relationship errors
Right Click ⟶ RelationShips ⟶ Add Persons to Family
Right Click ⟶ RelationShips ⟶ Create RelationShip
Right Click ⟶ RelationShips ⟶ Remove Person from Family
Right Click ⟶ RelationShips ⟶ Delete Family
Right Click ⟶ RelationShips ⟶ Merge

The context menu is helpful for fixing errors.
(Click on the image to enlarge)
 Refresh the chart using Right Click ⟶ Refresh, after making changes to relationships,
to see the changes.