title: Argument is not numeric or logical: returning NA Table with one column
body: <p>I've been searching other related questions but nome works for me.
  I'm totally new on R so please, speak to me like I'm 5. Also, english is not my first language.
  Now. I made a list on excel. Values between 2000 and 4000 and a header. My Dataset is named "Ejercicio 2.4".</p>
  
  <p><a href="https://i.stack.imgur.com/naLNB.jpg" rel="nofollow noreferrer">Excel table</a></p>
  
  <p>Just a simple list.
  Then, I imported the dataset to R like this:</p>
  
  <p><a href="https://i.stack.imgur.com/EkVoE.jpg" rel="nofollow noreferrer">Import Dataset</a></p>
  
  <p>I used the <code>summary(Ejercicio_2_4)</code> function to obtain simple stathistical data, but I wanted to usethe <code>mean(Ejercicio_2_4)</code> and I got the following message:</p>
  
  <pre><code>Warning message:
  In mean.default(Ejercicio_2_4) :
    argument is not numeric or logical: returning NA
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/JEE3G.jpg" rel="nofollow noreferrer">Warning message</a></p>
  
  <p>What am I doing wrong?</p>