The answer will depend on where you are running TurnKey Mayan-EDMS and how you launched it. FWIW most (if not all) of the details are all noted on the Mayan-EDMS page.

SSH username

The default username for SSH is 'root' - unless you launched from AWS Marketplace, in which case it's 'admin'.

SSH password

There is no default SSH password(s).

Password is set at launch or on firstboot/first login - depending on the platform you are using.

If you launched from the TurnKey Hub, then you need to either set a root password at launch time and/or provide an SSH keypair. You can either add a public key to the Hub prior to launch and all (future) servers, regardless of region will include it. Or you can select an existing AWS key - note that AWS SSH keys are region specific (so you will only be able to select from AWS keys that already exist in the region you are launching your server into). If you have already launched a server and didn't set a password or use SSH keys, then you could try to search the log for the randomly generated password, but it's probably easier to just destroy and launch a new one (being careful to set a password and/or set SSH keys).

If you launched from AWS Marketplace then as part of the setup process you would have had to either select an existing key registered with AWS, or create a new one (and download the private key and use that to log in).

Once you log in using your SSH keypair, you will be prompted to set a password.