The error happens because you use pruned mode, but want to load wallet.dat which never loaded previously or loaded after long time.
You can follow the Bitcoin Core suggestion (where you must download whole blockchain again) or simply extract the private key and load it to SPV wallet (such as Electrum).
But if you don't use pruned mode, please find debug.log (which located on folder where you store blockchain files & wallet.dat) and share it with us.
Thanks for that advice.
I don't know where the private key is at this stage. But I'm trying to avoid the process of hunting for it unless I can verify there is some coins in the wallet first. Is there a way I can extract the public key from the wallet?
It took nearly 4 days to download the blockchain, hoping to avoid that again.