Why am I getting "Cannot read properties of null (reading 'tagName')" error on checkout page in Magento?

This is a known error from the 3.10.3 version which was fixed in 3.10.4 as you can see in the release notes here: 

• Fixed the checkout console “tagName” error due to autocomplete not defined by ensuring proper loading of the js library

 
You can follow this guide outlining upgrading your Algolia extension.