Hi @Lisa_Crummett!
Yay!
The manifest format is something different than metadata in QIIME 2. Your manifest file was responsible for telling QIIME 2 about your data, so that it knew how to import it. The sample metadata file is about telling QIIME 2 about various study-specific metadata (treatment vs control, timepoints, environmental measurements, etc.) Some actions in QIIME 2 require metadata, while some don't, it depends on what the action is intending to do. feature-table summarize is an example of a visualizer that optionally accepts metadata.
The reason you got this error is because you provided a file that is viewable as metadata (rep-seqs.qza), but this data is oriented along the feature axis, while the summarize command is looking for metadata oriented along the sample axis. This error is specifically QIIME 2 complaining about not being able to find your sample IDs in your rep-seqs.qza file, which makes sense, because that file contains representative sequences, which is feature data.
Moving forward, you can rerun the command sans --m-sample-metadata-file, but I would recommend creating a sample metadata file, if it makes sense for your study. You can check out keemei, a tool we have created to help with setting up those files. I would also highly recommend going through the Metadata tutorial when you get a chance.
Hope that helps!