As someone who has been on both sides of the client/contractor relationship, often simultaneously, I thought it might be helpful to share my thoughts on the extremely unideal process of hiring freelance pixel artists via social media. There are certainly ways in which the client can improve on their half, but as the potential contractor is not the party in the position of power when they are actively applying for work (rather than being directly sought out by the client,) it is invaluable that they not stand in their own way while approaching the first hurdle. Actually completing the job should be what requires the bulk of an artist's time and energy, but there are relatively straightforward mistakes that I see artists making time and time again that will prevent them from ever getting hired in the first place.
1. Don't apply for the job if you are not at least somewhat qualified. Fake it till you make it can only take you so far, and as someone who is also an artist and has been at different stages in my abilities, I acknowledge that while there is a first time for everything, that first time should probably not be at the potential loss of the client. I am tremendously grateful for those clients that did allow me to create for them without tangible and direct proof of my ability to do so, and I hope that I was able to satisfy their needs. Art is challenging, and in my opinion it is relatively uncommon to go from start to finish on a project without feeling that discomfort at least a little. However, it is irrefutably true that simple confidence (feigned or ignorant or otherwise) will not make the job get done. Ultimately, you cannot fool the client, so don't waste their time or yours. An experienced client will likely see your true ability level, but even if not, a failed or subpar interaction will reflect poorly on you and will likely prevent you from getting more work in the future.
If you do believe you can complete the job, but this is not reflected in your portfolio or body of work, it is better to address this than to direct the employer to a portfolio of completely inapplicable material. There are blind spots in all of our portfolios, and that is fine. Ideally, you should prioritize adding these types of works to your portfolio at a later date, but for the time being you may suggest a paid art test to prove to both yourself and the client that you are capable of fulfilling their requirements. If the client is unwilling to pay this small additional cost, they're probably not a client you want to work with anyway.
2. It is always best to link to your work directly. Especially if your Twitter (or other social media account) is not used exclusively for art, it can be very easy for your work to get lost in all the memes and shitposting.
The early bird does get the worm sometimes. It is preferable to reply to the listing as quickly as possible, but if it takes a bit more time to compile your works before you press send, it is probably worth it. Please don't tell the client you will send them your art in the morning. While I know we all have various responsibilities in life, it is disrespectful to do this and by the time you wake up tomorrow the position will probably have been filled.
3. Flexibility is good, but if every single piece in your portfolio is in an entirely different style, subject matter, and even media, it makes me question if you are capable of consistency or have more than a little experience in any given area. There should be some thread of continuity throughout your portfolio in terms of the quality of your work overall, as well as evidence of your applied knowledge and your personal artistic eye. It's a good idea to have your portfolio reflect the work you want to continue to do or do more of in the future. This may mean both that you should remove works that are not in line with this vision, and that you should work towards adding more works that are.
4. On the other side of the coin, If you have an extremely limited range of style, the only way you can get hired is if the client wants or is open to adapting the project to your style. Generally, and especially when creating new assets for a previously existing property, you must be able to adapt to the style of the project. It's okay if your style doesn't work for every project— just don't apply for those.
The internet seems to value style consistency over personal growth and flexibility. Don't fall into this trap, because while you may grow in followers this may not be reflected in your resume or your wallet.
If, even after following this advice, you don't get the job, don't be too upset; there are a lot of pixel artists out there. There's a distinct possibility that whoever the client chose initially will show themselves to be incapable of completing the job (an all too frequent occurrence in my experience as a client,) or that the client will need more artists, perhaps with a different skill set, in the future. As long as you're somewhat qualified, it's always better to put yourself out there. You never know what the connections you make now will lead to down the road.