This is the client portal, not the cPanel login. Through the portal you manage your hosting services, domains and invoices. The credentials for the services themselves are different and will not work here.
Instead of your old username, you sign in to the portal with an e-mail address. If you had one account, it is the address that was its main contact. If you had several accounts whose main contact was the same e-mail address, each account got its own version of that address - with the username after a +:
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Your e-mail address
john.smith@gmail.com
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The username you used
agent10
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Try with
john.smith+agent10@gmail.com
Enter the address in that form and, if needed, use the password reset option.