While running Pipelines, there is an error that says "Validation error: This pipe requires a single item target, not a list"

I think we would need to see your exact pipeline and what it is doing. The only thing I can thing to check is what you are searching or trying to make changes to and see if it for some reason is being fed a list instead of a single object. You may need to do a loop or a bulk upsert instead.

Pipelines is very difficult and unintuitive to troubleshoot. Their activity log and all of that is barely functional when compared to other similar products like Workato. If this one thing was fixed for pipelines it would make a world of difference.