IME, the Marriott login function can be wonky on the new site.

If you are trying to login using the small window on the upper right corner of the main page, it will sometimes indicate unable to sign in.

If so, click on the menu "hamburger" on the top left, and select "login" or "my account" from there (I have seen both terms).
It takes you to a dedicated login page, which has always worked for me.