When you're using /fill and you're replacing a specific block, you can't place a block with a name containing an underscore (_). In my case I was using this command:

/fill 952 38 1021 894 43 1029 piston_extension 0 replace lapis_ore

I got the following output: Data tag parsing failed: Unable to locate name/value separator for string: lapis_ore.

The command did work after replacing "piston_extension" with something like glass or stone, but it also didn't work with blocks like gold_block.