How do I solve the VSTO word error ?

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C#

I am getting issue like this "Item with specified name does not exist."
This is the line of code that occurring error.

wordsec.set_Style(ref Text10); 
wordsec.set_Style("TableText9");
wordsec.set_Style("TableText8");

Here is code,

What I have tried:

Selection wordsec = Globals.ThisAddIn.Application.Selection;
 
if (sTextSz == "10")
{
object Text10 = "TableText10";
wordsec.set_Style(ref Text10);
}
else if (sTextSz == "9")
{
wordsec.set_Style("TableText9");
}
else
{
wordsec.set_Style("TableText8");
}

How can I solve this VSTO word error ? Please help me...