All the maelstrom commands in 6a 6b 6c are missing the leading ./ present in the other challenges.
The second command in 6b names the binary txn-rw-register but all other invocations name the binary maelstrom-txn.
The title of 6c reads " Challenge #6b: Totally-Available, Read Committed Transactions"
6b and 6c use -nemesis instead of --nemesis flag (probably both work, but --nemesis used in all other invocations)