Gary, sounds like you've got the wrong settings in there. Shouldn't be port 143, and according to their own published directions online, you're supposed to be using the mobile.charter.net server for everything (not just mobile).
https://www.spectrum.net/support/internet/adding-new-account-apple-mail/?redirected=trueIf you can't get the new settings to stick, you may want to try deleting the account entirely and then adding it back. You may want to back up the email locally before you do that, just to be 100% sure you don't lose it. (if you have regular backups, it will be there - but make sure you disconnect from the internet if you find yourself having to actually USE that backup; otherwise it could connect and sync with Yahoo online, which will override the local version; you have to drag things INTO the inbox/sent mail folders so that the local version is synced to the server, not the server saying "nothing in MY inbox, therefore there should be nothing in YOUR inbox" on initial connection). Note: the likelihood of you actually losing your email by deleting your ACCOUNT (not the mail) locally is very low, but it's not a good idea to tempt fate.
Another thing you should try, perhaps, if changing the settings doesn't work, is to try creating the Mail account on a test user account OR you could download and try Thunderbird (free) and see if it can get your mail ok. Either one of these exercises, if successful, point to something wrong in the settings in Mail in the current user accounts, which is my suspicion anyway, given the error message Colette received. It points to the wrong mail server and wrong port being used to fetch mail. Port 143 is the default unencrypted port for IMAP mail; Yahoo doesn't allow it.
I'm not surprised to find that Spectrum/Charter are using Yahoo for their mail services; same as Rogers here in Canada. In my experience, it's always been my Rogers accounts that cause problems; Gmail and my domain email just keep ticking along. I've been trying to decide whether or not to just stop using the Rogers account, but the problem is, I've been using it for 12 years and soooo many things are tied to it.