Don't forget about your vault! If you don't have enough space to keep your money safe,  you won't be able to accumulate more than a certain quantity (you will start with a 1,000 limit). You can tap directly on the vault or on the stats button (upper-right corner icons), and there you will see the price and requirements to expand its capacity. 

 

 

 

Please, check this old FAQ to learn how to check the requirements.