Cannot open file " file name.xlsx " because the format or extension of this file is invalid

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C#

I have a project where I am using form1 having gridview and form2 containing textboxes. My code runs fine and saves the file to the directory that I choose. The problem is when I try to open the excel file I receive an error from Excel stating “Excel cannot open the file Invoice System.xlsx because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file

What I have tried:

form1:
using (var fs = new FileStream("Invoice System.xlsx", FileMode.Create, FileAccess.Write)) { wb.Write(fs); }
form2:
private void button1_Click(object sender, EventArgs e) { string[] rowData = new string[] { textBox1.Text, textBox2.Text}; form1.dataGridView1.Rows.Add(rowData); }