The Arizona Motor Vehicle Division (MVD) notes that AZ law requires under certain circumstances that the owner of a vehicle must carry a SR22 for three years. So if you are listed on the title of the vehicle (due to being a co-signer) that is probably the reason the state of Arizona is requiring you to hold a SR-22 certificate of finance responsibility.
Without knowing your specific situation that would be our educated guess as to why Arizona wants you to also carry a SR-22 though you are not a driver of the car or policyholder of insurance on the car. To find out the exact reason you are being required to file a SR-22 you need to speak with a representative of the MVD or whatever AZ state agency that notified you that you need to carry a SR-22.
When you contact the MVD ask if there are other options for you since you do not drive or carry insurance on this vehicle and only are a co-signer on it. If you are required to file the SR-22 due to your name being on the title or registration of the vehicle than you may need to see if there is a way in which to take your name off of it so that you do not have to carry this insurance due to your relative's issues.
You may want to seek expert legal advice with an Arizona lawyer that is familiar with this type of situation and can help you find out if you have any other options.
If you must carry the SR-22 certificate and your current insurance provider does not file this proof of insurance with the state than you will need to get your car insurance coverage with an insurer that does.