When attempting to check or uncheck a checkbox in Service Portal using the ENTER key on the keyboard, this does not affect the checkbox at all.

1. Enable Accessibility via System Settings.
2. Navigate to the Service Portal and perform a search for a keyword in knowledge articles, e.g. "mail": /sp?id=search&spa=1&q=mail
3. On the Filters on the lefthand side, click and expand the Category.
4. Using the SHIFT key on your keyboard, cycle through the elements on the page until you're hovering over a checkbox.
5. Press the ENTER key on your keyboard to try check or uncheck the checkbox.

Observe the checkbox is not affected by pressing the ENTER key on the keyboard.

This is expected behavior and by design. The ENTER key does not toggle the checkbox state across standard platform browsers and on the native UI as well.
As a workaround, use the SPACEBAR key if you are intending to check or uncheck via the keyboard.