In short the IRS was effectively closed for almost 3 months and the priority was to issue the stimulus checks first, then refunds and lastly processing returns with balance dues ... which may not be processed until 7/15 or later.
You can pay online even if you don't see a balance due on the system ... or mail them a check. Either way your payment will be put on your account as a 2019 payment on the balance due ... make sure to choose that option online or write that info on the check.
