Are you trying to set up a POP or IMAP account? When you say Spectrum is to forward your mail, that would imply that one email account is sending emails received to another email account. Is this what you are trying to achieve? Have you been using the connection settings on the web page published by Spectrum?

It is IMAP. Yes, I get email from Spectrum. but I don't want to go through the steps of signing in every time I want to check my emails. I want to forward this email to Thunderbird if I can have it on desktop as my email client. I have followed all Spectrum's directions and nothing has worked. When I test the settings, I get a message about a connection not being able to get through. I never had email issues when using outlook classic...but then Microsoft shut it down without warning and a lot is messed up.

Having Thunderbird forward emails to a different email address would be created by setting up a filter by accessing the Hamburger icon (top right)/Tools/Message Filters. That's for future reference...

...but I understand you to say that you want Spectrum to send emails received to Thunderbird. So, that would have to be set up in Spectrum. But that would also be a POP account, not an IMAP one.

IMAP mirrors the contents of the Email Server while a POP account downloads the emails from the Server.

So, I'm a little confused as to what you're trying to achieve. It doesn't take much to confuse me, either...