ERP implementation projects can go off-track for a number of reasons.
It's quite easy for ERP projects to go off-track given the number of 'problem' scenarios below:
Wrong selection - this is probably the worst case scenario as no-one wants to admit they made a mistake and recovering from the wrong selection is a long and costly process. Hopefully this is not your problem.
Inadequate Statement of Work (SoW) - this is very common as it is challenging both for clients and suppliers/partners to get SoWs completely right when there are wide knowledge gaps at the start of an implementation. If you are experiencing SoW scope creep then the best thing to do is halt the project and do the detailed analysis needed to re-issue the SoW and revise solution, budget and timescale expecations to the new reality.
Deficient project execution/management (internal) - your team or PM is not good enough or your internal PM practices not robust enough. Needs someone who knows what they are doing to get a grip of your project.
Deficient project execution/management (external) - your partner team(s) or PM is not good enough or their PM practices not robust enough. Needs reading the riot act to your partner and firing their PM to force improvement or switching partner altogether (always a last resort).
Development cost overruns - development costs are always a big contributor to cost overruns so all 'customization' needs to be managed through a formal change request (CR) process and all billing to the project linked to change requests otherwise no payment is made.
No idea what you are aiming for - both clients and partners should know what the 'minimum viable solution' (MVS) actually is. If this has not been defined up-front via a Target Operating Model (TOM) or a set of SAT (Solution Acceptance Test) 'run-the-business' processes then it is hardly surprising that projects go off-track when neither party understands what the destination is.
No idea what is core to solution success - similar to above but specifically related to not understanding what the core differentiators are in your business that will make the difference between a solution that adds value and one that does not. This should have been established in the selection and if not then via the SoW analysis work. If neither then your selection and/or SoW was deficient.
Deficient RAID management - if you have made poor assumptions, do not understand project dependencies, do not monitor risks and allow issues to get out of control then your project will invitably suffer. This is the job of the steering team to manage.
And these are in addition to other unexpected project impacts like BREXIT, COVID, staff shortages etc. that can hit any project at any time.