Is it okay to say?
<div>Your live streaming seminar overlaps with my lesson, therefore I can't attend.</div><div><br></div><div>It doesn't sound natural, does it?</div><div>How can I say it better?</div>

<div>"Therefore" is a little formal.  I'd say:</div><div><br></div><div><em>I can't attend your live streaming seminar, because it overlaps with my lesson.</em></div><div><br></div><div><em>I won't be able to attend your live streaming seminar, because it overlaps with my lesson.</em></div>

<div>Usually we use the word "coinicide" to replace the "overlap".</div><div><br></div><div>Just my 2 cents worth.</div>