Short answer
You are ready when you have an executive sponsor with real authority, a team that can give the project meaningful hours, and agreement on why you are changing. You also need data you understand well enough to move.
I usually tell finance directors to look at people before they look at technology, because the project will need your best payroll, AP and budget people for a year or more and somebody has to backfill their day jobs while that happens. The second thing I look at is whether leadership agrees on what success looks like, since an ERP project with three different definitions of done (one from the manager, one from finance and one from IT) will drift no matter how good the software is. Data is the third piece, and that means knowing how many years of history you need to bring over, who owns the vendor and employee master files, and how clean your chart of accounts really is. None of this has to be perfect before you start, but if you can answer those questions honestly you are in a strong position, and the Start at Zero Navigator includes a readiness checklist you can work through with your team.