The "code" of a store is the code that Magento will use to call that store. As the message suggests, you should only use a combination of letters, numbers and underscore symbols to input the code. Were you using any other symbol when filling in the code?
This Community forum is intended to solve problems related to the Bitnami Stacks, Virtual Machine or Cloud Images but your question is very specific to Magento as an application.
For information regarding the application itself or customization of the content within the application, we highly recommend checking forums and user guides made available by the project behind the application. You may try asking that question in the Magento Forums.
Thank you for your understanding.