This article talks about the solution for the "updater application is not available" issue you might face when trying to update/install Magento using the Web Setup Wizard.
Issue
The following message is displayed in the readiness check:
Affected products/versions
- Magento Commerce 2.2.x, 2.3.x
- Magento Open Source 2.2.x, 2.3.x
Solution
To resolve this issue, see if there is a <magento_root>/update directory that contains files and subdirectories. Otherwise, see Set up the updater.