i have this too on one of my sites but not the other
often hosting companies disable this php function for security
the bit in the code that uses it is likely not needed,
you can prevent the error by renaming file create-tables.php to create-tables.php.bak after plugin activation
this file is only used to create the database tables the plugin needs on installation, by renaming it, it stops the bits of code runnning that use the exec function.
