The tag should be {service.solusvm_password} in the Welcome email template. The module re-uses an existing SolusVM account for each client, so the first time you create a service for a client, the password will be available, but for additional services, it is not. For example, if you add another SolusVM service to a client that already has one, then the existing SolusVM account is used, and the module does not know the password that was created for a previous service, and even if it did, it may have changed.
Usually for cases like this, we setup the email template to say something similar to:
{% if service.solusvm_password %}
Password: {service.solusvm_password}
{% else %}
Your password is the same as your previous SolusVM service.
{% endif %}