Consistenly getting Block magic mismatch error? I have bitcoind running (0.18) in my ubuntu machine and it keeps shutting down due after getting this error:

ERROR: ReadRawBlockFromDisk: Block magic mismatch for CDiskBlockPos(nFile=0, nPos=68821043): 00040000 versus expected f9beb4d9

I restart it and it goes for a couple of hours and then the same error is encountered and it shuts down.  How can I fix this?
You have a corrupted block file. You will need to delete that file and all higher number files and then redownload the entire blockchain.

The block file that is corrupted is the 0th file, blk0000.dat. Since this is the first block file that Bitcoin Core creates, the easiest thing for you to do is to just delete the entire blocks and chainstate folders in your Bitcoin Core datadir. Then start Bitcoin Core. It will redownload the entire Bitcoin blockchain.