You don't need any reputation to write an answer, however, you do need reputation to add comments to posts. You can always post comments on your own posts or answers to one of your questions.
Comments are meant for transient information such as improvement suggestions, clarification, or minuscule information. This also means that comments will be removed when the relevant information was incorporated into the posts in order to declutter the Q&A. As such, the privilege to create comments is only extended to members of the stackexchange, whereas it is a privilege earned very early.
If you wish to answer questions, please post an answer.
If you have a new question, please create another topic. Should it be closely related to another topic, please link to the previous question if it helps to provide context and also explain why the linked question doesn't solve your problem.
In your specific case, you posted an answer requesting additional information from an asker. This answer was turned into a comment by a moderator as it wasn't answering the question but requesting clarification. However, the asker never returned to provide more information and the question was purged by the system after nine days of abandonment.
By purging abandoned content the system helps the site to improve signal-to-noise ratio and improve content quality. The creators of StackExchange explained their reasoning here: Optimizing For Pearls, Not Sand
To put it crudely, our site aims to provide rather a small number of great questions and answers that are relevant to many users than an endless stream of low-quality content relevant to few.
If you have specific ideas where we can improve our content, please feel free to provide input—we could definitely use more help.