user23516: I just want to add black video at the beginning of the video 
I have found following commands.
ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref=1*iw/1/sar:ih/1[wm][base];[base][wm]concat=2" output.mp4

MainQuestion:

How can I solve this issue?

user1871: concat requires videos to have the same sample aspect ratio as well as the same resolution.
So, in this case,
ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref[wm][base];[wm]setsar=1[wm];[base][wm]concat=2" output.mp4

or
ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref[wm][base];[base][wm]concat=2:unsafe=1" output.mp4


23516: <p>I just want to add black video at the beginning of the video </p>

<p>I have found following commands.</p>

<pre><code>ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref=1*iw/1/sar:ih/1[wm][base];[base][wm]concat=2" output.mp4
</code></pre>

<p><strong>MainQuestion:</strong>
<a href="https://i.stack.imgur.com/WR6RI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WR6RI.png" alt="enter image description here"></a></p>

<p>How can I solve this issue?</p>
 1871: <p>concat requires videos to have the same sample aspect ratio as well as the same resolution.</p>

<p>So, in this case,</p>

<pre><code>ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref[wm][base];[wm]setsar=1[wm];[base][wm]concat=2" output.mp4
</code></pre>

<p>or</p>

<pre><code>ffmpeg -i s1.mp4 -f lavfi -i color=c=black -t 10 -filter_complex "[1:v][0:v]scale2ref[wm][base];[base][wm]concat=2:unsafe=1" output.mp4
</code></pre>
