Hello Venkat,
Could you please check if necessary entries are maintained in t-code PIDE of ECC system for direction R/3->CRM and CRM->R/3?
Also, please check if there are filters for object CUSTOMER_MAIN (R/3->CRM) and BUPA_MAIN(CRM->R/3) in t-code R3AC1 in CRM system.
Thanks,
Best regards,
Julin