For security, Ghost blocks repeated login attempts with the wrong credentials to prevent people from trying to hack your site.
If you see this error, you will need to wait before trying to sign in again. The more times you enter incorrect credentials, the longer you will have to wait.
If you've forgotten your credentials, you should instead click on "Forgot" to reset your password by email.
If you are using Cloudflare, you may incorrectly trigger this error due to the Cloudflare caching settings of your domain. You can set up a PageRule to disable caching on your Ghost Admin routes like this: