Undefined function in fresh install?

I’ve been having a problem with my media browser for weeks and haven’t been able to solve it. When browsing to insert an image, the media library shows absolutely nothing, just blank white. I see tabs telling me that it sees the objects in the library, but they do not display as thumbnails to select.

For some reason it just occurred to me (a-doy) to look at my server logs – turns out my ISP has been suppressing PHP errors. The error I’m getting is this:

PHP Fatal error: Call to undefined function get_post_thumbnail_id() in REDACTED/wp-admin/includes/media.php on line 1292, referer: https://REDACTED/wp-admin/media-upload.php?post_id=877&type=image&

Now this is baffling – I did a completely fresh install of WordPress to try and solve this problem, and yet I’m still getting an “undefined function” error. Why is this happening, and how can I fix it?

Any help is greatly appreciated. Thank you!