iChat was a messaging app that supported other protocols, so that Yahoo Messenger, Google Talk, AOL Instant Messenger, etc. could all be accessed from one tool.
However, iChat was discontinued in macOS Mountain Lion 10.8, where it was replaced by iMessage, now known as Messages.
Messages replaced the 'other protocols' with a proprietary, end-to-end encryption system for communication among Messages users, but also supported SMS, incorporating this new mobile phone messaging standard, allowing non-Messages users to be included. Messages does not support Google Talk, AIM, etc.
More critically for you, Google announced back in 2017, that on June 16, 2022, they were discontinuing support for third party access to Google Talk. Therefore your use of iChat and Google Talk is no longer supported.