White spaces are required between publicId and systemId

The error message is actually correct if not obvious. It says that your DOCTYPE must have a SYSTEM identifier. I assume yours only has a public identifier.

You’ll get the error with (for instance):

<!DOCTYPE persistence PUBLIC
    "http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">

You won’t with:

<!DOCTYPE persistence PUBLIC
    "http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" "">

Notice "" at the end in the second one — that’s the system identifier. The error message is confusing: it should say that you need a system identifier, not that you need a space between the publicId and the (non-existent) systemId.

By the way, an empty system identifier might not be ideal, but it might be enough to get you moving.