Linux mint 18.3, convert ext4 to btrfs
Solution 1:
I suppose you should start with the current btrfs documentation. The good news is that conversion from ext4 is indeed supported, but tutorial you linked did indeed skip some steps listed in the linked wiki manpage. You should also be able to follow the instructions on the Arch wiki for this.
The error message should be benign, in the sense it should show the error but allow you to boot . See for example here (Mint, 2011) and here (manjaro, 2016).