Firstly, in histories_T, you are referencing table T_customer (should be T_customers) and secondly, you are missing the FOREIGN KEY clause that REFERENCES orders; which is not being created (or dropped) with the code you provided.
There may be additional errors as well, and I admit Oracle has never been very good at describing the cause of errors - "Mutating Tables" is a case in point.
Let me know if there additional problems you are missing.