Here are a few steps you can try to resolve the issue:

Wait and Retry: Often, this error is temporary. Wait for a few minutes and try making your purchase again.

Clear Cache and Cookies: Sometimes, clearing your browser's cache and cookies can help resolve such issues.

Use a Different Network: If you're on a shared or public network, switching to a different network might help as the rate limit could be applied to the entire network.

Contact Steam Support: If the issue persists, reach out to Steam Support. Provide them with the details of the error and what you were doing when it occurred.

Remember, Steam has systems in place to prevent abuse, and sometimes legitimate users can get caught by these measures. Patience and a bit of troubleshooting usually resolve the issue.