Why Can’t I Proceed with an Upgrade in Idle Supermarket Tycoon?

Whenever you get to unlock something new in the game (e.g. new department), you are relatively free to upgrade it as much as you want from that point of purchase -- onward. Probably the only bottleneck you'll encounter when upgrading, is when it costs a lot to purchase upgrade especially when compared to the rate that you are earning (which then tells you that you should move to the next city). If you can't push through with an upgrade, the only plausible reason is that you do not have enough (or right amount of) resource to purchase the upgrade -- especially that the value indicator (of you resources) only displays abbreviations of the actual amount.

For instance, Sextillions will have a value indicator of "s", when Septillions will have "S". It is important to take note of the small and capital letters as they are technically different values. The same is true with Quadrillions and Quintillions, Decillions and Duodecillions. Regardless of the case, the one with the capital letter always denotes the higher value/amount among the two. As a quick guide to help you figure out the sequence of the values, we've listed them below:

Millions

Billions

Trillions

quadrillions

Quintillions

sextillions

Septillions

Octillions

Nonillions

Decillions

Undecillions

Duodecillions

Tre-- etc

quad-- etc

Quin-- etc

sex-- etc

Sep-- etc

Oct-- etc

Non-- etc

Dec-- etc

Un--etc

Duo-- etc

Confusing right? During the first few cities you'll be playing-in, you might get confused with these values -- but eventually, you'll get the hang of it as you move from city to city. The rule of thumb is that you will always have to take note of the value you currently have -- if it is in uppercase or lowercase and you'll be good to go.