wfLoadExtension( 'UserMerge' );
// By default nobody can use this function, enable for bureaucrat?
$wgGroupPermissions['bureaucrat']['usermerge'] = true;

[6cc061292b8db5d3b3f89303] /index.php/%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:UserMerge CannotCreateActorException from line 2516 of C:\xampp\htdocs\math.local\includes\user\User.php: Cannot create an actor for a usable name that is not an existing user

Backtrace:

#0 C:\xampp\htdocs\math.local\extensions\UserMerge\includes\MergeUser.php(378): User->getActorId(Wikimedia\Rdbms\DatabaseMysqli)
#1 C:\xampp\htdocs\math.local\extensions\UserMerge\includes\MergeUser.php(50): MergeUser->mergeDatabaseTables(string)
#2 C:\xampp\htdocs\math.local\extensions\UserMerge\includes\SpecialUserMerge.php(135): MergeUser->merge(User, string)
#3 C:\xampp\htdocs\math.local\includes\htmlform\HTMLForm.php(660): SpecialUserMerge->onSubmit(array, OOUIHTMLForm)
#4 C:\xampp\htdocs\math.local\includes\htmlform\HTMLForm.php(552): HTMLForm->trySubmit()
#5 C:\xampp\htdocs\math.local\includes\htmlform\HTMLForm.php(567): HTMLForm->tryAuthorizedSubmit()
#6 C:\xampp\htdocs\math.local\includes\specialpage\FormSpecialPage.php(184): HTMLForm->show()
#7 C:\xampp\htdocs\math.local\includes\specialpage\SpecialPage.php(569): FormSpecialPage->execute(NULL)
#8 C:\xampp\htdocs\math.local\includes\specialpage\SpecialPageFactory.php(558): SpecialPage->run(NULL)
#9 C:\xampp\htdocs\math1solutions.local\includes\MediaWiki.php(288): MediaWiki\Special\SpecialPageFactory->executePath(Title, RequestContext)
#10 C:\xampp\htdocs\math.local\includes\MediaWiki.php(865): MediaWiki->performRequest()
#11 C:\xampp\htdocs\math.local\includes\MediaWiki.php(515): MediaWiki->main()
#12 C:\xampp\htdocs\math.local\index.php(42): MediaWiki->run()
#13 {main}