Error using Synaptic in KNOPPIX_V8.6.1-2019-10-14-EN.iso unpacked as persistent

[rajibando]

As usual I use unpacked KNOPPIX_V8.6.1-2019-10-14-EN.iso and booted with Grub to create a persistent system. In general, I have long been using synaptic to install additional software persistent across multiple reboots.
With the current version, I have run into an issue. I get the notice from synaptic:

E: The value 'stable' is invalid for APT::Default-Release as such a release is not available in the sources
E: _cache->open() failed, please report.

I checked the /etc/apt/sources.list
The keyword 'stable' shouldn't be an issue, but is presently. Shall the keywords be replaced by exact words? Like stable=buster, oldstable=stretch, unstable=Sid, oldoldstable=jessie and so on?

[rajibando]

[Solved] Use apt-get update as a super-user to have synaptic working