This answer expands upon the answer given by @ScottWelker.

There are some quarks when working with Azure DevOps and numbered headers.

If you want to refer to a heading with a number like

## 2.1 Heading
Add a bunch of filler here to space out the page.

You would expect to write

[2.1 Heading](#2.1-heading)

However this will first try to link to a DevOps item using #2 and render it as

[2.1 Heading](.1-heading)

which is no good.

What you need to do is to add a path to your current document (as if you were linking to an anchor in a separate Markdown document) and instead write

[2.1 Heading](./<link-to-your-current-md>#2.1-heading)

which will actually create the correct link to the numbered heading anchor.

Be aware that this is only the case for DevOps Markdown - for normal Markdown you would usually write

[2.1 Heading](#21-heading)

as normal Markdown will strip .s and replace spaces with - dashes.