Most Likely Cause and Fix
Your markup contains a form with a blank
What This Error Means
Some element attributes can take pretty much any value, others can take only values of a certain type and others still can only take a value from a pre-defined set.
This error relates to the following usage in your markup:
The error is stating that, in this context, the attribute
form is not allowed to take the value
The validator is giving the reason: "must be non-empty."
How To Fix It
For form actions
Add a relevant value for the
<form action="http://example.com/" … >
<form action="" … >