Please make sure you enable Bluetooth access.
If you don't see this option, please try the steps below:
Go to iPhone Settings > Screen Time > Turn on Content & Privacy Restrictions > enter Bluetooth Sharing and select "Allow Changes." > Check whether Plaud appears in the list and whether the button is on.
If the button is not on, please turn it on and then check whether you can see the Bluetooth option for the Plaud app and turn it on to try again.