An error about the schema

What pipeline are you using.
The Passthru pipeline has no components and does not attempt to identify the message.
The XMLReceive pipeline contains the XmlDisassembler which will attempt to match the incoming Xml data with a deployed schema. It uses the root node and target namespace as the message type and this should match a schema. If the schema is not found the pipeline will stop and generate an error.
If you are using flat files you need to create your own pipeline containing the Flat File Disassembler and specifying the schema that you wish to use to parse the incoming data.