Can not open messages in lockdown mode

What is Lockdown Mode?

Lockdown Mode is an optional, extreme protection that’s designed for the very few individuals who, because of who they are or what they do, may be personally targeted by some of the most sophisticated digital threats. Most people will never be targeted by attacks of this nature.

When Lockdown Mode is enabled, your device won’t function like it usually would. To reduce the attack surface that could potentially be exploited by highly targeted mercenary spyware, certain apps, websites and features will be strictly limited for security, and some experiences may not be available at all.

For the majority of end-users, Lockdown mode will neither be useful nor helpful. When active, much of an iPad’s functionality is completely disabled. 

More information about Lockdown Mode can be found here:

About Lockdown Mode - Apple Support