Anchors in posts generating errors

When I try to add an anchor in a post or page, whether within the same post/page or to a specific part of a separate post/page, the link always generates the following error when clicked:

Fatal error: Call to undefined function get_header() in /full/path/to/site/files/wp-content/themes/one-day-at-a-time/index.php on line 1

When using a relative URL, it seems to tack the #anchorname to the URL for the index, as opposed to the index for the post/page containing the anchor.

Any thoughts on what might be causing this and how to resolve it?

Site: https://www.cats3dayadventure.com

Thanks in advance for the help.