Receiving error message "The type "XmlNode" is defined in an assembly that is not referenced.", but appropriate assembly is referenced

Question

I've been searching for solution for a long time with no luck. I'm using this code to extract text from a .odt file:

public String ReadOdt(String path)
        {
            var sb = new StringBuilder();
            using (var doc = new TextDocument())
            {
                doc.Load(path);

                XElement stylesPart = XElement.Parse(doc.DocumentStyles.Styles.OuterXml);
                string stylesText = string.Join("\r\n", stylesPart.Descendants().Where(x => x.Name.LocalName == "header" || x.Name.LocalName == "footer").Select(y => y.Value));

                var mainPart = doc.Content.Cast<IContent>();
                var mainText = String.Join("\r\n", mainPart.Select(x => x.Node.InnerText));

                sb.Append(stylesText + "\r\n");
                sb.Append(mainText);
            }
            return sb.ToString();
        }

However, Visual Studio is telling me:

Error CS0012: The type 'XmlNode' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Xml, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. (CS0012) (ScriptTablerV0)

And the same thing for type 'XmlDocument'.

As a solution it proposes to import the System.Xml reference, but this doesn't change anything and the errors persist.

Is there anything i should look at or try? Any help would be greatly appreciated.

EDIT: I've tried using other methods/classes from the System.Xml assembly and they've worked with no problems, but VS still doesn't recognise Styles and Node.