This is a technical artifact and can be ignored as a non-specific warning.
To clarify: the input to Unique.seqs does not contain any data? This means that some upstream step was problematic (usage), or none of your data actually passed the filtering criteria applied by Screen.seqs (or possibly another upstream step/tool). Not all tools will “fail” (turn red) when given empty inputs – some instead just produce more empty inputs.
Traceback through your analysis to find out where the problem was introduced – and it wasn’t necessarily the most recent upstream steps. The metagenomics GTN tutorials that include Mothur tools can help with usage and example data/workflows that are known to work.
More help here, but to be clear, the issue you explain is a tool usage or data content issue, not a tool/server issue (if I am understanding correctly!). Why there are empty results from the upstream tool should be addressed first.
Thanks!