Looking at the code it looks like the email field they are inputting is going to field 142, and there isn't a field 13 on this form. So most likely there is another email field that is required in QB that isn't being included in the data being pushed to QB and that is blocking your save and throwing you that error.
<label>Email:
<span class="small">Enter your Arcosa email address</span></label>
<input type="text" name="_fid_142" id="email" class="texta">
<div class="spacer"></div>
------------------------------
Evan Martinez
Community Marketing Manager
Quick Base
------------------------------