To understand which shipping provider was not found, the message must be opened.
You will then find, for example, this message:

Object
message:"Carrier ID 123 not mapped in OTTO market config."
trace:Array[3]
code:0
line:339
Trigger:"backend: POST /plenty/admin/gui_call.php"

This error indicates a missing configuration of the shipping provider stored in the order (Order > Tab: Settings). In this example, it would be ID 123.
The ID corresponds to the shipping provider ID under Setup > Orders > Shipping > Options, Tab: Shipping Providers.

In the Otto Market Setup Assistant, it must now be checked whether the shipping provider stored in the order also has a valid mapping.
Usually, this then needs to be updated.

Afterwards, the shipping should be checked again.