TL;DR; Check that your Domain Controllers are in the correct OU and that Microsoft Key Distribution Service is running
I ran into an issue recently when DNSSEC signing a dns zone where Windows Server 2019 gave a very vague error, and would only display that error after 10 minutes of timeout. This made iterating on it very slow since every change I made was a 10 minute wait. Every guide to setup DNSSEC mentioned right clicking the zone, then clicking sign and as long as you select the default it should just work. On another domain, that happened for me and it just worked; except the one original one that kept timing out.
In setting a custom DNSSEC signing policy I noticed that there were different keystores each of which gave a different error. This made me think it was something to do with the specific one I was using. It was time to troubleshoot the service itself not DNSSEC.
I got a list of the services from a known good, and signing, domain controller; then compared that to the bad one to see what was different. Part way down the list I noticed that Microsoft Key Distribution Service was failing to start, and if I tried to start it, there was an error.
Group Key Distribution Service cannot connect to the domain controller on local host Status 0x80070020.
Checking the Event Log showed an issue in finding the Domain Controllers on the network (error above), which was weird because it is a Domain Controller… In looking at where this system was placed in the domain tree, I saw it had been moved from the original OU for domain controllers to another place. I dragged it back, after applying all the GPOs that were on that other folder to the original Domain Controller folder. Then held my breath, hit start on the Key Distribution Service and it started right away.
After that DNSSEC signed with no issues. Long story short, dont move your DCs it’ll only end in pain. And to the one other person on the internet who has seen this problem and never solved it, 5+ years ago https://www.reddit.com/r/sysadmin/comments/3dedwm/dnssec_will_not_sign/ there is your answer!