The issue occurs when trying to add a new component to a page of a non-SXA site and create a new datasource for it. The Create button in the Create New Datasource dialog does not exist, but it is available in SXA version of the dialog.
The issue can be experienced only if SXA 1.8.0 or a later version is installed, and does not occur in Sitecore XP solutions without the SXA module.
To resolve the issue, download and install the patch or cumulative hotfix compatible with the affected product version found on the following pages: