Hello,
To set/select the particular size guide to particular category, first you have to turn ON the Size Guide to show on Single Product Page Options from Theme Setting Product Page as shown in this screenshot: https://prnt.sc/p0mvau and then this option exists when you create a size guide, on the right side you can select which product categories to display this table on, as shown in this screenshot: https://screenshot.net/ved45t9 and save changes, Hope it will work for you.
If you still have the problem, send your site admin access, so can check it for you on your site and let you know the cause of the issue on your site.
Best Regards.