Sorry, I don’t know what happened to your site. The login info you given is not working anymore. I cannot enter your site to investigate.
Please turn off WP_DEBUG in your wp-config.php, it’s for programmers to look at when we need to debug issues, turning it on can sometimes cause conflicts.
Please use the following code for your bootstrap button, it includes a target and is working properly.
<a class="button" href="https://google.com" target="_blank">Learn more</a>