If you happen to face this error message "Copying pre-existing data completed with error, For additional details, go to initial sync details tab" while starting the dual-write entity map then please check you have setup the cross company data sharing policy for the underline table of the entity map (which is a data entity in D365 FO). To resolve this issue, Go to System Administration | Setup | Configure cross=company data sharing screen and look for policy where you may have setup the underline table of the data entity used for dual-write entity map. Disable the policy and then run the dual-write job again, wait until it turns to running state. Enable the cross-company data sharing policy again choose No on the next open pop-up window otherwise it will run against all data across all legal entitles which might not be necessary for this exercise as you just disabled it to start dual-write entity map.