SOLVED: “This record type cannot be deactivated because the following profiles use this record type as default.”
I just thought I’ll play with a Record Type for Opportunity and delete it later but got an headache finding how to solve this error. Finally, here is the way:
Goto Setup -> Administration Setup -> Manage Users -> Profile
Go inside your corresponding profile and search for that particula Record Type name, in my case it was “Partner Opportunity Record Type”. Edit this and remove not required record types and go back and try to deactivate the record type. Should work now.