Walmart returns a wide range of error messages, and it's not always easy to tell exactly what's wrong with your listing. In most cases, the issue comes from one or more fields on the Category tab of your listing.
The specific fields involved vary depending on which category you're listing within, so the example below may not match your exact error message. Even so, it illustrates the most common reason this type of error happens.
Example: Jewelry category fields
If you're listing in Walmart's jewelry category, you'll see a "Chain Length Measure" field and a "Chain Length Unit" field. Neither field is required on its own, but it's good practice to fill in as much information as possible.
Here's the important part: if you provide a value for one of these fields, the other becomes required. If you tell Walmart your product's chain length is 16, Walmart needs to know what unit that refers to, inches, feet, or something else. Likewise, if you specify inches as the unit, Walmart needs to know how many inches. Filling in only one of the two fields triggers an error, since Walmart can't interpret an incomplete measurement.
Similarly, entering something like "16 inches" directly into the Chain Length Measure field also causes an error. This field should only contain a number. The unit itself (inches, feet, and so on) belongs in the separate Chain Length Unit field.
Other common cause: description length
This error can also appear if your listing description exceeds Walmart's maximum of 4,000 characters.
How to fix it
Check the Category tab on your listing for any paired fields (like the chain length example above) where only one side has a value. Fill in both fields, or clear both if the information isn't relevant to your product. Also confirm your description is under 4,000 characters. Once corrected, republish the listing.