An error occurred while installing the selected updates.

Try this:

Repair the Drive for El Capitan or Later

1. Restart the computer. Immediately, at or before thechime hold down the Command and R keys until the Utility Menu appears.
2. Select Disk Utility and press the Continue button.
3. Then select the indented (usually, Macintosh HD) volume entry from the side list. 
4. Click on the First Aid tab in the toolbar and wait for the Done button to activate. Click on it, then quit Disk Utility.
5. Select Restart from the Apple menu.

Now, try updating again: Download macOS Mojave 10.14.6 Combo Update.

How much storage is remaining on your startup drive? If it is under 20 GB, you will need to free up storage in your home directory before proceeding with the Mojave update. The recommendation about Disk Utility First Aid is quite valid too.

I have the exact same thing, downloads 300mb then stops, useless. Also now my apple app stopped delivering my yahoo mail

I fixed the problem by going straight to apple and downloading the update then clicking on the update to install it.