Device PINs can only be used on one device (tablet or phone) at a given time. So, for example, if you've used one device to create your account and test Surveyapp and decide you want to use another device to conduct the surveys instead, you won't be able to sign in on the second device with the same PIN as your first device - unless you sign out of the first device. (Or you can set up the second device as a new device at https://app.surveyapp.io/devices.)
You can sign out of the first device either from the app, or you can edit the device at: https://app.surveyapp.io/devices. Simply click on the device name, and under the PIN tab, switch the device status from 'IN USE' to 'AVAILABLE' by clicking on it and save your changes.
You will now be able to use the PIN on your second device.