An estate plan generally requires a general durable power of attorney but banks and other financial institutions may refuse to honor general durable powers of attorney as the New York Times reports in "Finding Out Your Power of Attorney Is Powerless."
The issue for the banks is that they fear being held liable if they accept the power of attorney document and it turns out that the person given access to a financial account is acting improperly. When someone shows up with a power of attorney and requests access to an account, the bank does not necessarily have a way to determine that the account holder is actually incapacitated.
Because of this concern, many financial institutions will not accept a standard power of attorney form and instead insist that their own house forms be used.
There are a couple of things you can do about this.
Before you become incapacitated, ask financial institutions that you do business with what their requirements are. If they require their own forms, then fill them out.
An elder law attorney can give guidance if a bank refuses to honor your request.
Reference: New York Times (May 6, 2016) "Finding Out Your Power of Attorney Is Powerless."