I cannot add an additional email account on my MacBook Pro

I cannot add an additional email account in Apple Mail on my MacBook Pro (OS 15.6.1).

Since Spectrum is my ISP, I obtained a Spectrum email account from them. I checked it out, and it works fine (sending and receiving) using a browser.

I would like to access Spectrum email in Apple Mail (where I have two other POP email accounts). I followed the prompts to add an additional email account. However I continue to get an Apple Mail error message saying that the password or email is wrong — and they are not.

I do not have an iPhone connected, so this is just for my MacBook Pro.

Please help!