After installing and configuring ORDS 3.0.10, I noticed that APEX ran fine, but when testing the sample HR REST module I received the following error:
The request could not be mapped to any database. Check the request URL is correct, and that URL to database mappings have been correctly configured
The reason was that the table ords_metadata.apex_pool_config was missing the column pool_name. To fix this, I simply had to run the ORDS validation:
java -jar ords.war validate
This added the missing column, and the error went away.
I found the answer to this on OTN.