Please check the following for No ‘Access-Control-Allow-Origin’ header is present on the requested resource exception in Salesforce Chat.

1. Check whether “Permitted Domains” have any URLs. If yes, then chat can be initiated only from those URLs.

Note:

Please do not use https:// in the Permitted Domains. 

2. If the issue is in your Test site connected to a Sandbox, check whether the Sandbox was recently refreshed. If yes, replace the code in the website with the new Chat code snippet from Salesforce.

3. Do not use iFrames.