The following error appears to occur when _apt user does not have access to the downloaded .deb file.

This could indicate that the folder 
'/root/assets' does not have readable permission for other users. You could try fixing by following methods 
1) Move  Dungeondraft-1.0.4.7-Linux64.deb to a folder that is readable by _apt. You could copy it to /tmp for example, and install from there and it will get rid of that warning.
2) Change the access permissions of the folder you are installing from. To perform so in a linux environment use following commands:
> sudo chmod +r /root/assets

This will grant other users read access.

3) You can try changing ownership of the folder to be owned by '_apt'. To perform so in a linux environment use following commands:
> sudo chown -Rv _apt /root/assets

If the above don't work it could also be a bug within the package manager. Check with the latest bug reports online to see if the running version may have already encountered similar issues.
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