When you don't have enough Tokens to upgrade a part. You can spend Coins to purchase the missing Tokens and upgrade the part.

This "Coins price" varies based on how many Tokens you currently have to upgrade a part. Therefore, if you only have a few Tokens missing to upgrade a part, then, you will only have to spend a few Coins as compensation for the missing Tokens.

That's why you cannot spend only Coins to upgrade parts as it represents an extra cost in addition to the Tokens you have.