Screw Up With Site Transfer

Not certain whether there is help in my regard, but I’ll extend my white flag. I thought I would be able to do my own site transfer from one server to the other, but I’m not sharp enough.

Here is the link https://steelercentral.com/ and the errors

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘_show_post_preview’ not found or invalid function name in /home/steelerc/public_html/wp-includes/plugin.php on line 406

Fatal error: Call to undefined function get_post_format() in /home/steelerc/public_html/wp-includes/post-template.php on line 337

I replaced plugin.php and post-template.php, I’m unable to log-into my wp admin panel to address the plug-ins ‘unable to make database connection’

I’m just not learned enough, any suggestions? I may simply switch the DNS back to the original server, more trouble than it’s worth thus far…

Thanks