Trying to use Nik Collection and I don't see a way to do that within the subscription version of Lightroom.
No, that feature is not in Lightroom CC. It is in Lightroom Classic CC, but has not been integrated into Lightroom CC at this time.
What Jim said.
See table of differences between CC and Classic CC here
For example, your question, under Extensibility: Third Party Plugins shows: Classic CC "Yes", CC "No".