unable to update or install plugins after migration from subdomain to main

I have manually migrated my site from subdomain to main domain. I copied all the files and sub directories to public_html folder . in main domain a demo site of wordpress was running.
after copying all files to access WP admin area I edited wp-config file added WP_HOME and WP_SITEURL entries with new domain name.
I logged in wp-admin sucessfully
then I installed new plugin “velvet blues update URL” and updated all URLS.
I checked my new site and it was working file no issues with posts and pages.
Now the problem came when I tried to install a new plugin or update any plugin it is not working. It says unable to copy file. for example for one plugin update it gave the following message
Update failed: Could not copy file. elementskit-lite/modules/library/init.php

but the site is working file and I able to edit pages etc. problem is only with updating plugins or installing a new one.
Please help me where I have gone wrong while migrating



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