"VST Sound library "VST3 Presets for Flux" not found"

Maybe this won’t help, but I hope it will… It looks a lot as the issue I stumbled on, recently :

.vstsound file missing : a little (real-life) tale, with what could be a “how to ?” procedure… - Cubase - Steinberg Forums

EDIT - Beside this, the path shown in your error message might be a cause of it : no .vstsound file should be installed in a …\Downloads location…