Thank you for confirming the Project.
You will need code to provide the logic to let the app know how you will 'spend' coins ...You will most probably make extensive use of the If..then else or else if blocks.
My suggestion is to take this one piece of a time. Save the app you have now as an aia to keep it save. Then start several projects with different names to code the buying logic and a way to save information to the TinyDB. Your project is not difficult but it is complicated. Building parts, then later connecting them may save you lots of time. It is easier to debug a small demonstration app then your main game app that may have many blocks. Make a chart on a piece of paper showing yourself how you want your app to work; it will help you to design what you need to code. Again what you want is complicated, not difficult if you have done the tutorials.
Try some blocks. If you have questions, post them as a new thread and include an image of what you have done (jpg or png images are best) and you will get more assistance.
Good luck.
Regards,
Steve