I went ahead and left an answer below with the payment steps. I suspect you are choosing the wrong tax year to print, which will result in a loop like you described and take you back to the Tax Home page. It is not intuitive. You have to choose "This Year's return" in order to be presented the payment screen since you are paying for the 2018 return. See the full explanation below.
Since you said you are already in PLUS, I didn't include the steps on how to upgrade. I'm assuming you are in PLUS and already know how to open the return up. So I merely provided the correct payment steps. Whatever you do, do not upgrade any farther, or you'll have problems. See my full answer below.
Also, for this to work, the 2017 and 2018 returns MUST be in the SAME account. So I'm assuming they are.
