is it illegal for a potential employer to train you for 12 hours and not be paid?

I originally wanted to work for this boba shop and the manager said that once I complete training and was accepted into the job role, I would be paid for the duration i spent training. I changed my mind today and do not want to further pursue this job as the wage would be little after train tickets. Meaning that if I were to no longer want to take this job role, I would receive no money.
However, someone said to me that it is illegal to work at a shop and not be paid? should i ask for payment or is this not the case?

I assume there's nothing in writing - like a contract of employment, or similar?

What you agreed to do sounds like a "trial period" after which the employer might, or might not, decide to offer you a job. Unfortunately, there is no clear law in this area, only guidance.

I suggest you take a look at the "Unpaid work trial periods" section on this page about the national minimum wage. It tries to draw a distinction between whether or not it was a genuine ‘trial’, ‘test’ or ‘recruitment exercise’ - for which the employer would not need to pay you the minimum wage - or whether you should have been considered a 'worker' - in which case the minimum wage would apply.

Have a read and see which better reflects what happened during those 12 hours. There are some examples in that section too.