Hi there,
I have checked your description. To check the error I need to know whether it is built in custom PHP or Wordpress. Generally you need to make sure that the information for the database name, username, password, and database host is correct. Otherwise it create issues or errors. Might be your reason is same as I have mentioned or different.
I will be obliged if you consider my formal offer and allow me to have a small discussion session with you.
Best Regards,
BDM (Sd)