Why does adding new fields to my .env file on Laravel lead to a 'command_not_found' message in the CLI?

The errors are actually coming from the shell, not Laravel.

The issue is in your .env file. You have spaces around the = sign, like this:

SERVICEUSERNAME = "user"
SERVICEPASSWORD = "password"
SERVICESITE = "sandbox.example.org"

Laravel’s dotenv parser might still be okay with it, but sail also loads .env through a shell context. In shell syntax, this is not valid, so it ends up being read like SERVICEUSERNAME is a command. That’s why the error looks kind of weird.

So, basically, just remove the spaces:

SERVICEUSERNAME=user
SERVICEPASSWORD=password
SERVICESITE=sandbox.example.org

SERVICEUSERNAME="user"
SERVICEPASSWORD="password"
SERVICESITE="sandbox.example.org"

After that, run:

sail artisan config:clear

And if the Sail/container environment still feels like it’s caching the old values, restart it:

sail down
sail up -d

Also, small note: this PHP string looks a bit sus:

"baseuri" => "https://{config('services.service.subdomain')}/api",

'baseuri' => 'https://' . config('services.service.subdomain') . '/api',

Or this one:

'base_uri' => sprintf(
    'https://%s/api',
    config('services.service.subdomain')
),