Is the billion dollar bill spendable?

No. It is fake money and is not "legal tender" or real money.

Any supposed "US bill" with a denomination higher than $10,000 isn't real. There are loads of novelty "million dollar" and "billion dollar" bills that sell for a few bucks at gift and trick shops.

There were real $100,000 bills issued in 1934 and 1935 but they were only used for making transfers inside the Federal Reserve System; none were put into circulation. All but a couple of them were later destroyed by the Treasury. These bills are often counterfeited, especially in the Philippines and other East Asian countries.