I once worked with a client when I was working for a NS partner. After the whole implementation, they said nothing is useful. We told them you signed the SOW. They said "yes, we signed it, but it's not what we wanted" and I was so surprised to see their response.

get sign-off every iteration

Well I have left them long ago... and they also got shut down, I believe... or lost almost all major clients.

That means they didn't do proper requirement gathering, analysis or requirement elicitation. Best practice in to run requirement workshop with client's every department effected by the ERP to come up with a base line requirements - then use that to come up with optimized ones. Also need to dissect all the business processes, if needed shadow them while they are performing them. I remember when worked as a consultant in some cases spent days in the client's warehouse or shop floor to see their operation in action - sometimes bosses don't have clear idea of the whole process to begin with, so how can they give you a definitive set of requirements.

Recently had a project where they were convinced the warehouse was running way more simply than it actually was. Got off the phone with the warehouse manager and went "well, we're underscoped here" that was a fun conversation, but definitely had it before starting to build things out.

Fortunately, @Sam-I-Am I was not the one who gathered requirements. When I joined, the project was half-way through the implementation. Some customizations were needed. I was heading the technical side and following the SOW. The person who said it's not what they wanted was the one who signed off on the scope document/SOW. But the company was not running the practice properly either. So it was their fault as well.