Base64 command is case sensitive

Replace the capital D with a lowercase d for the base64 command. See below

$ aws iam get-credential-report –output text –query Content | base64 -d

$ base64 –help

Usage: base64 [OPTION]… [FILE]

Base64 encode or decode FILE, or standard input, to standard output.

With no FILE, or when FILE is -, read standard input.

Mandatory arguments to long options are mandatory for short options too.

-d, –decode decode data

-i, –ignore-garbage when decoding, ignore non-alphabet characters

-w, –wrap=COLS wrap encoded lines after COLS character (default 76).

Use 0 to disable line wrapping

–help display this help and exit

–version output version information and exit

The data are encoded as described for the base64 alphabet in RFC 4648.

When decoding, the input may contain newlines in addition to the bytes of

the formal base64 alphabet. Use –ignore-garbage to attempt to recover

from any other non-alphabet bytes in the encoded stream.

GNU coreutils online help: https://www.gnu.org/software/coreutils/

Full documentation https://www.gnu.org/software/coreutils/base64

or available locally via: info ‘(coreutils) base64 invocation’