What version of InDesign are you using? Build Number is located on the About InDesign menu item.
So the PDF's tag tree isn't constructed correctly. Most common cause:
The <Document> tag is not at the top of the tag tree. This tag should contain every other tag in the PDF file. Example:
If you don't have a <Document> tag, make one (add it to the tag tree) and drag all the other tags into it.
Some of the tag tree's tags are not in the <Document> tag. Drag them into <Document>, making sure to retain their correct reading order.
You have some crazy nesting of tags, such as a <Table> inside a <Hx> tag. Or a list that's not tagged correctly.
This type of error is caused by incorrect construction of the original InDesign layout file. In my classes, I stress how to "engineer" the construction so that these errors are eliminated. I can't see your InDesign file so it's difficult to guess what you're doing in InDesign to cause the problem. But the most common causes are related to incorrectly threaded frames to create a story, Paragraph Styles not used consistently, and hard returns either splitting paragraphs or missing entirely.
Hope this gives you some ideas where the error is occuring.
But that's a band-aid to get you past the checker, and not the best way to make a PDF fully accessible to everyone who reads it. Always best to master the accessibility methods in InDesign so that these errors don't get built into the PDF from the start.