Hello @Tushar Talele
Thank you for reaching out to the Community regarding your issue, although I am sorry to hear that you are experiencing issues with American express. Do you know the exact day you started experiencing the issue? Has anything in your account recently changed that could cause error? Another thing here is have you taken a look at the CC-503 article just yet? If not I would recommend seeing trying the recommended steps to start off. I will leave the article down
https://www.quicken.com/support/error-when-updating-accounts-cc-503
Once you get a chance to try these steps, please let us know what you find. From there we'll better understand our next steps.
Thanks,
Quicken Francisco