Coap receive messages

Hello team,
How can I receive messages via coap from the emqx-broker using the “get” function.
The topic used has been created and subscribed to.
Allways i received the error code 400 “invalid observe value”.
When I send a post message to the topic, I get the same message back because this topic is subscribed.

How can I actively query the subscribed topic with a message without overwriting the last value?

best regards