You should first make sure Bluetooth is turned on and try connecting your device in the Bluetooth settings. If your iPhone still wont connect to Bluetooth, you can try deleting other devices from Bluetooth settings, updating your iOS software, resetting your network settings, or restarting your iPhone entirely.

How do I get my iPhone to automatically connect to Bluetooth?

How to Auto Connect an iPhone to a Bluetooth DeviceStep 1: Move iPhone Near The Accessory. Bring the iPhone to within a few feet of the Bluetooth device.Step 2: Turn iPhone and Accessory On. Step 3: Accessory Pair Mode. Step 4: Open Settings App. Step 5: Enable iPhone Bluetooth. Step 6: Select The Device.