Sorry for the delay in response.
As the scanned document is not uploading to the Document Cloud, please make sure that the internet connection is not intermittent.
Also, check if the "Allow cellular data usage" is enabled under the preferences of the application. Take help of the steps mentioned here App Settings — Adobe Scan Help.
Adobe Scan does not save the scanned PDF in the local storage of the device.
When you scan the document and click on Save to PDF, the PDF is uploaded and saves in the Document Cloud.
You can attach the scan only after it saves as the PDF in Document Cloud.
Once the scanned document is saved as PDF, open the document in the application.
Then click on the Share icon at the bottom and select "Share a copy".
Refer this help link Scanning Documents — Adobe Scan Help.
Let us know how it goes.
If you experience any issue, feel free to update this thread.