Q: Use laptop as Bluetooth speaker

bluetooth speakers for anything else then phone call use the bluetooth profile called a2dp which both iPads and iPhones support read more

https://en.wikipedia.org/wiki/List_of_Bluetooth_profiles#Advanced_Audio_Distribu tion_Profile_.28A2DP.29

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iOS: Supported Bluetooth profiles - Apple Support

make sure the computer is put in pairing / discovery mode if that is required, pretty sure that few bluetooth stacks on computers actually support being a a2dp headset so maybe you're lucky with your laptop

from the iPad and iPhone it just see an a2dp bluetooth device it's transparent for it if it's a headset, external speakers or in this case a laptop or car

also make sure the computer does not have an active connection with the iPad when you attempt to pair