To access a player you can either use
Where 0 means the first member in the party. or
Where 1 means the first actor in the database
So in the conditional branch you can run the script:
$game_party.members.sp >= $game_party.members.maxsp / 2
if $game_party.members.sp >=
$game_party.members.maxsp / 2
p 'Over half SP'
p 'Under half SP'
And I'm not really sure how you planned on implementing luck, if you wanted to use variables I suppose you could do something like
Control Variables: [0001: Actor1 LUCK] = 50
Control Variables: [0002: Actor1 MAXLUCK] = 100
Using the control variable command and then with code
luck = $game_variables
maxluck = $game_variables
if luck >= maxluck / 2
p 'Feeling Lucky!'
p 'Feeling Unlucky'
Or once again if it were in a conditional branch
$game_variables >= $game_variables / 2