Entity not defined
The document uses a named entity, like  , that XML does not know.
The message, as the validator reports it
Entity 'nbsp' not defined
Why it happens
- HTML entities copied into XML:
,©,é,—… XML defines only five:<,>,&,"and'. - An entity declared in a DTD that is not loaded.
How to fix it
Replace the entity with the character itself (the document is UTF-8 by default), or with a numeric reference:   for a non-breaking space, © for ©. If the entity belongs to a DTD, declare that DTD in the document.
Example
Reports the error
<p>Price: 10</p>
Fixed
<p>Price: 10</p>