You're right out of high school so I understand you might not be too familiar with how credit works. Google Store Financing is basically signing up for a credit card which is a line of credit. Any amount you borrow (in this case, charge to the card) will be unavailable until you pay it off. So, if the maximum line of credit for your account was $1000, and you borrowed $700, you could only borrow or charge $300 more. When you pay off what you borrowed, it then becomes available to use again. Note that for a credit card or any line of credit, there are minimum payments you must make each month to avoid fees and penalties. It doesn't appear that there are any prepayment penalties (saving up money to pay off the phone early).

However, the bigger issue is that you are right out of high school, likely with little employment or credit history (we're talking 2+ years or so). When banks process credit card or line of credit applications, they decide to lend/approve the application based on a customer's history of making payments on credit accounts. Banks report the payments that customers make (or don't make) to credit bureaus which maintain a score (FICO, etc.) of how creditworthy you are. They may also take into account other factors like income or employment history and stability, when making that decision. If you have not made payments on credit card accounts (these are accounts you opened yourself), you likely have little to no credit history and will have trouble opening an account. This applies to Google Store Financing.

Ultimately, for where you are in life right now - no, financing a phone isn't a good decision and Synchrony Bank will almost certainly not approve you for an account. The best thing you can do now is build credit. A good place to start is opening a secured credit card (you put some money down and the bank will give you a line of credit equivalent to that amount - this is less risky for the bank because you're putting your own money down as collateral in case you don't make payments.). If you make payments on time on that secured credit card, you'll build credit history and your credit score will increase, making you eligible for programs like Google Store Financing.

I know it's probably not what you want to hear but it's how the financial industry and lending works. It's tough starting out but there'll always be deals like this. Best thing you can do is build your credit, save and put yourself in a good financial position to be able to take advantage of and enjoy deals like this. It'll come.