Can bitcoin be hacked? This is a broad question, and there are many places where hackers can enter the internet, and gain bitcoin by stealing it. The actual bitcoin blockchain is very difficult to hack, as the cryptography that is used would take a tremendous amount of power to actually hack the bitcoin blockchain.
That said, there are many vulnerabilities. Many of the wallets that are commonly used for bitcoin storage, especially hot, or connected, wallets are relatively easy to hack (compared to the blockchain itself).
Exchanges are at constant thread of being hacked, and there are organized groups that hackers who specifically target exchange wallets that contain millions in bitcoin, and other tokens. Individuals should also be very careful with their connected wallets, and always use a backup wallet that isn't connected to the internet.
Another way that hackers get illicit bitcoin is tricking their victims into sending bitcoin under false presences. For example, some hackers target exchange websites, and create bogus addresses for unwitting exchange clients to send bitcoin to, instead of the legitimate exchange.
Professional bitcoin custodians use cold storage, which means holding the private keys for the bitcoin in an offline computer, or some other form of disconnected device. Once the private keys are offline, it becomes much more difficult to hack the system.