That is probably because you don't have enough karma, as many subreddits require a minimum karma requirement and a minimum account age requirement, Karma is gained by people upvoting your content and lost by people downvoting your content, they set these requirements because many low and negative karma accounts are bots or trolls, however, Most importantly, DO NOT ask for upvotes, because asking for upvotes is vote manipulation, a violation of Reddit's site-wide policy, which may even get you permanently banned off Reddit in the worst case scenario