I had the same issue.
To resolve it – I downloaded the latest version of WordPress from here: https://www.ads-software.com/download/.
I unzipped the downloaded folder and copied the functions.php file from wp-includes. Using FTP I pasted the new functions.php into the wp-includes directory onto my web hosting, when asked if I wanted to overwrite existing file I said yes.
I now have access to the Dashboard and my blog.
