Increasing the number of div by one each time

There are a couple of issues in your code. Firstly, you have an extra dot after the ToString() method call, remove dot from the last. Secondly, the usage of the index i inside the XPath might not work as intended. Here is a revised version of your code with proper string concatenation using string interpolation to dynamically inject the value of i into the XPath expression:

for (int i = 0; i < dataGridView1.Rows.Count; i++)
{
    var val1 = dataGridView1.Rows[i].Cells[3].Value.ToString();

    driver.FindElement(By.XPath("/html/body/app-root/app-auth/app-public/app-card/div/div/div[6]")).Click();
    driver.FindElement(By.XPath($"/html/body/app-root/app-auth/app-public/app-card/div/div/div[6]/div[{i+2}]/div/div[1]/kendo-datepicker/button/span")).Click();

    Thread.Sleep(100);
    driver.FindElement(By.XPath($"/html/body/app-root/app-auth/app-public/app-card/div/div/div[6]/div[{i+2}]/div/div[1]/kendo-datepicker/button/span")).Click();
}