You will have to modify the below line:
<li><a href="#" data-toggle="modal" data-target="modalRegister">Register</a></li>
modalRegister
is the ID and hence requires a preceding #
for ID reference in html.
So, the modified html code snippet would be as follows:
<li><a href="#" data-toggle="modal" data-target="#modalRegister">Register</a></li>