On Wednesday - same date as the OP - I was trying to book an award ticket. The Web site showed me the fares/routes/dates/etc. as normal, but when I tried to book tickets, there were errors.
The first time, the error message as to "Correct the two items" in order to book, but there were no items marked as needing correction and I failed to find any entry errors after going over the page twice. I cancelled that effort, opened a different browser and tried again.
Essentially, the same result, except this time the error was "This cannot be done on line" and instructed me to message or call Delta. The phone agent was able to get the flights I wanted and attributed the problem to a "software glitch," which seemed obvious to me. I was more interested in what kind of glitch and what they were doing to fix it.
Technical details: On delta.com, route was DTW-MEM (trying to burn miles and need to go visit my mother) in coach for travel about a month out. Browsers were Firefox and Chrome. Still running Windows 10 on my laptop, if that matters (have to get a new laptop to upgrade to Windows 11...haven't seen the need for that yet).