What mistakes do orgs commonly make when implementing ERP?

Don’t previous review their processes and orchestration of the activities, adapting, as much as possible, their business routines to the ERP’s best practices.
Moreover joining the master data management in a unique shared service center that will take care of the main data files, cleaning, consolidating, unifying, verifying and assuring the accuracy of those records is another milestone a lot of organizations fail to do.

Inadequate Testing and Quality Assurance: Insufficient testing and quality assurance processes can lead to system glitches, functional gaps, and performance issues. Thorough testing, including user acceptance testing, is essential to ensure that the ERP system functions as intended and meets the organization's requirements.

Allowing too many requirements and scope creep.  It needs to be a staged roll-out and communication regarding expectations. 
   

Bringing in too much or not enough historical data

Not getting company side buy in. Installing comphrensive systems like this requires full buy in by th e primary stakeholders in each department that will be using it. When these projects fail
It’s because departments don’t give accurate requirements / input to the solution to ensure it’s configured correctly and then when it rolls out they refuse to change their ways to adapt to the ERP.