A field cannot be renamed or merged into another field while it is being used by any assets in the system - where do you find what is using that field?
- Go to the Admin section in Marketo
- Click on Field Management in the left sidebar
- Locate the field through the Search on the right
- There is a "Used By" section that shows the forms, smart lists, and smart campaigns, and any other assets using that field
- Go to those assets and remove the field name from it