You will not be able to create new tags on your first few posts - this is to keep the number of tags from getting out of control, to try to encourage use of existing tags and to avoid too much overlapping.
However, if you feel like we need a particular tag which doesn't exist yet, simply reply to this message and tell us what tag you would like, and how/when it will be used, and we'll add it to the list.
But please think about your tag before suggesting it. The tag should be descriptive and meaningful. It should be specific enough to differentiate enough from other tags but not be too specific – it should be useful for other challenges, projects or resources.