This error is generated in wp-login.php , it happens if server is unable to set cookies, this can happen due to several reasons, one of the common issue is: output being sent before setting up of cookies.
Try:
– deactivating ALL plugins. If you don’t have access to your Dashboard’s Plugins page, try manually resetting your plugins (no Dashboard access required). Here is another tutorial: https://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/ – If that resolves the issue, reactivate each one individually until you find the cause. Also remember to deactivate any plugins in the mu-plugins folder (if you have created such folder). The easiest way is to rename that folder to mu-plugins-old.
– switching to the default (twenty Nineteen) theme. If you don’t have access to your Dashboard’s Appearance page, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel (consult your hosting provider’s documentation for specifics on these), navigate to /wp-content/themes/ and rename the directory of your currently active theme. Alternately, you can remove other themes except the default theme (Twenty Nineteen, etc.). That will force your site to use it.
– MANUALLY updating. Download WordPress again and unzip it, and use that to copy up. Access your server via SFTP or FTP, or a file manager in your hosting account’s control panel (consult your hosting provider’s documentation for specifics on these), and delete then replace your copies of everything on the server EXCEPT the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. (NOTE: do not delete the?wp-content?directory or your?wp-config.php?file). Please read the Manual Update directions first.
If the above trouble shooting steps do not help, then check error logs or, try Enabling debugging and post the results.
