When single sign-on (SSO) is enabled for your user, your account does not have a an associated password. Therefore, your username and password do not work with external clients to check out your repos or make a commit. Instead, use your username and an API key with repo access.
To get an API key
Click your avatar, and then select My Profile > API Applications & Sessions.
Check the Repository access box, and then click Create.
Once your API key with Repository access is created, you can view the API key and secret on the page. In a client or command line, you can use your Assembla username and your API Secret (in place of a password) to authenticate with your repositories.
Need help? Contact us at support@assembla.com.