Since the indicated value of --dump-privkeys arg isn't an absolute notnor relative path,
If not in your BTCRecover nor python install directory, it should be in your working directory named "blockchain-github-v1-1_main_privkeys.txt".
About "working directory", it's the directory displayed in your command line before the ">" sign.

Or alternatively, specify the absolute path of where you want to save the dump file.
e.g.: --dump-privkeys D:/blockchain-github-v1-1_main_privkeys.txt to save it in D: drive's root.

If it's not the password, it may be the payload that you saved as wallet.aes.json file.
The format should look like the test "blockchain-github" wallets in ".\btcrecover\test\test-wallets" (open as text), older versions are different than the latest so check them all if your payload isn't formatted correctly.

Fingers Crossed.
But like I mentioned, his suggestion to dump the wallet's private keys isn't the direct solution to this but an option in case it's the client's issue.

Since the indicated value of --dump-privkeys arg isn't an absolute not relative path,
If not in your BTCRecover nor python install directory, it should be in your working directory named "blockchain-github-v1-1_main_privkeys.txt".
About "working directory", it's the directory displayed in your command line before the ">" sign.

If it's not the password, it may be the payload that you saved as wallet.aes.json file.
The format should look like the test "blockchain-github" wallets in ".\btcrecover\test\test-wallets" (open as text), older versions are different than the latest so check them all if your payload isn't formatted correctly.

Fingers Crossed.
But like I mentioned, his suggestion to dump the wallet's private keys isn't the direct solution to this but an option in case it's the client's issue.