Please make sure you read official WordPress detailed instructions here: https://codex.wordpress.org/Installing_WordPress#Detailed_Instructions
After reading official instructions, above instructions will hopefully make much more sense.
Please let me know if that helps.
This is because WordPress cannot connect to MySQL database. A change in database credentials can result in a such problem.
I suggest checking your credentials are correctly set in wp-config.php. If you did not modify this file, I suggest you login to cPanel / Plesk, and reset your database credentials.
If you’re not sure how to do this, I suggest you ask your host for help.
