Migrating WordPress to Plesk isn't just about copying the site's folder. You need the complete files, the database, the correct connection details, and a plan for switching the domain over.
Prepare the files and database
Back up the WordPress files and export the database. On the new hosting, create the domain, a database, and a database user.
Test the site before changing DNS
Check panel login, forms, media, internal links, and email before the domain starts pointing to the new hosting.
Switch the domain and check SSL
After changing DNS, set up SSL and check both the www and non-www versions again.