"Agent has no remaining credits. Renew your subscription or buy a credit pack to continue." means this period's credits are used up. The agent takes no new messages or tasks until you add credits.
Pro plans: Manage, then Add Credits, buys a pack of 5, 10 or 20 times the monthly credits at the plan's rate, paid the same way as the plan. Credits are added as soon as it's paid. Starter plans can't buy packs.
Upgrade to Pro for a larger monthly pool (see upgrading).
Wait for the renewal, which refills the monthly credits.
To make credits last: choose a cheaper Agent Model under Agent Settings, pick a smaller model for single messages, or start a new chat session (see agent pricing). Where the credits went: Logs and Transactions.