What are flexible jobs that a beginner actress can obtain when they are on their down time
While it likely sounds cliche', the reason a lot of aspiring actors take serving or bar tending jobs is because they can get shifts that start later in the day; which leaves time for auditions during daylight hours. Retail is another option but you can run into lack of flexibility depending on where you work.
You can learn a skill that allows you to schedule clients or students for the times of your choice, and get paid by the hour. For example, if you go to massage school for two years, you can get certified as a massage therapist and do that part time, out of your home or by renting space. You can decide on specific hours of the week when you'll take clients. Or, if you play a musical instrument, go to a music school for a couple of years, learn how to teach that instrument, and you can find a few music students who will go to your home once a week for lessons. If something comes up you can often reschedule with them. Or, if you develop your language and writing skills, you could be a writing tutor for high school or college students. Many times parents will pay for tutoring for their teenagers. There are other skills you can turn into flexible part time work based on your own schedule, these are just some examples.
I don't know anything about acting, but I have a few suggestions for jobs that are flexible and may be able to fit your needs.
You may want to look for jobs that people can do remotely, on their own time. For example, there are some reputable survey websites where you get paid a very small amount for each survey you take. There are also companies doing market research that sometimes hire people to go into stores and take notes on their experience. These are just small examples, but this type of remote job might be what you want to look for and/or ask a guidance counselor about.
How close are you to where you'd have to audition? Assuming you're pretty close, you could also look into part-time evening jobs, like waitressing, retail, or working at a movie theater, if your auditions would all be during the day. These might be more lucrative than the specific examples I posted above (surveys, market research), but other remote jobs may pay better.
Try to look for "survival jobs" which have some flexibility built into them (waiting tables, some shift work jobs, book stores, some retail stores, etc) that will allow you to change Your hours on short notice as long as you find a "cover" for yourself. Don't lie about where you're going or what you need the time for - only creates more problems down the road. Also try registering with Temp Work services. They can often help match you up with a job or an employer who understand your situation and is willing to work with you sorting your job and audition schedules. And, stick with it - just be up front about everything.