This message is displayed if you are trying to connect to a server that has a higher security level set than the default setting of 8. You have 2 options to connect to such a server:
Option 1: Import your already existing identity from your desktop computer (which might have an already raised security level)
Option 2: Create a new identity on your desktop computer and raise its security level, then export it to your iOS device
Visit this link to get more information on how-to import your identity to your iOS device.