whenever i paid the remaining interest of nation's debt it shown gateway timeout (The gateway did not receive a timely response from the upstream server or application. Additionally, a 504 Gateway Timeout error was encountered while trying to use an Error Document to handle the request.)
so suggest me, what should i have to do to pay the remaining interest of nation's debt

Can you provide the exact steps you are taking when this occurs?

Can you provide screenshots of the steps?

first of all i clicked top of right on treasury,
then in tax summary on details
pay now
enter amount
pay debt now
then it occurs gateway timeout.