Concatenate string to the_field()

Question

You’ll have to show more of the relevant code, but, to begin with, if you want to check for the availability of a ‘preparation_time’ ACF value, use

if ( get_field( 'preparation_time' ) ) : // get_field(), not the_field()

As the ACF docs put it, the_field( 'field' ) is the same as echo get_field( 'field' ) – so wouldn’t return true or false, but instead simply print (if you get to it).

I’d probably write it

if ( get_field( 'preparation_time' ) ) {
    echo get_field( 'preparation_time' ) . 
        ' 40 MINUTES PREP/COOK' ;
}

But no guarantees, since I don’t know the context.