You're not signed in to the AWS account root user. Only someone that's signed in as the root user can close the account. If you can't access the root user account, then contact the root account user and ask them to close the account for you. If you have the root user account credentials but can't sign in, then see I forgot my root user password for my AWS account.
If you use a mobile device, then use a desktop browser instead.
Important: Your account permanently closes in 90 days, after which you won't be able to reopen your account and AWS will delete the content remaining in your account. To reopen your account before your account is permanently closed you must contact AWS Support as soon as possible. Also, within 60 days from the date of account closure, any outstanding balances must be fully paid, including providing any information specified on the invoice.
Note: For privacy and security purposes, AWS Support can't close accounts on request.