It looks to me that the
schema.xsd file that controls the URN schema validation for the new
db_schema.xml files is either missing or you have not cleared the cache.
First, try removing all folders and files inside the
var/cache folder and run the
bin/magento setup:upgrade command again. It that doesn’t solve the issue, check that you have correctly upgraded all Magento files by checking if the following file is present in your installation:
If that file is missing, you will have to try upgrading again your Magento installations. If you are using the downloaded zip package, just copy again the files and be sure to replace the existing files in your installation by the new ones. If you are using composer, follow these instructions: https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html