What version of core do you currently use?
The latest official 22.0 release.
Is it full sync?
It is regtest, there are no connections, I just put the next block after Genesis by myself.
In regtest it matches the minimal difficulty, in other networks it has to be mined first.
I've followed this exactly but I didn't encountered any error.
Try also importing Satoshi's public key:
importpubkey "04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f" "satoshi"
I deleted all data files and executed both commands, then I reached the same error again. So maybe it works only if someone imported Satoshi's key.