That's a really hard question to answer, because you haven't provided us with any information that we can work with. In that case, all we can do is to take guesses and hope something works. Here's my guess:
A Bluetooth device can be paired with up to 7 other Bluetooth devices at a time. So my guess is that your ear buds are already paired with 7 Bluetooth devices, meaning you have to unpair it with at least one device.