Multiple issues.
The error given by framework is a generic one and your interpretation is incorrect. Refer what Atanu has to say.
My pointers -
1. You have not specified the Entity set name in service URL.
2. If you are using PUT method (Updating data) specify the key also as part of URL something like
<service URL>/<Entityset name>(<Key Value>)
Example -
<service URL>/SalesOrderSet('1212121212')
BR.