Loaded variable can't be indexed, unrecognized function or variable error

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I am attempting to pull a specific index from data loaded into a file. Here's the code in question:

load 'aes_power_data.mat';  
n_traces = 200; 
traces = traces (1:n_traces, :); 
plaintext = plain_text (1:n_traces, :);
...
    
function [result] = predict_Sbox(key, p_Text, byte)
    PT = plaintext(p_Text, byte);
    pos = bitxor(key,PT);
    result = sbox(pos);
end

Load places a 200x16 double into the workspace, but the line "PT = plaintext(p_Text, byte);" is when the error occurs. So the first time I call this, it should pull the first value from plaintext (1,16). What am I doing incorrectly? Is the error in catching the variable or in my use of function or something else?