Defining conversions return type declaration is only available in php 7 only

[poria92]

Hi guys
I use PHP 7.4.6

and in Defining conversions

public function registerMediaConversions(Media $media = null): void
{
$this->addMediaConversion('thumb')
->width(368)
->height(232)
->sharpen(10);
}

I got this error
"return type declaration is only available in php 7 only"
how do i handle this.