Cannot install program from the Synaptic Package Manager

I tried to download and install the Pixillion Image Converter Software from the Synaptic Package Manager. It downloaded and started to install. Then the message below appeared.

W: Download is performed unsandboxed as root as file ‘/var/cache/apt/archives/partial/python3-problem-report_2.20.4-0ubuntu4.8_all.deb’ couldn’t be accessed by user ‘_apt’. - pkgAcquire::Run (13: Permission denied)

I am using Lubuntu 17.04 and Mozilla Fiefox

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