Hey there ptj77,
These casting issues can be pretty tricky to figure out, so I'd be happy to help you resolve this! Just to confirm, are you only having issues casting from your Chromebook, or are you unable to cast from any device? Regardless, the first thing I would recommend is that you follow the steps listed in this article. If you run into any error messages, let me know, and I'll be able to troubleshoot a bit deeper.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!