Struct contents reference from a non-struct array object

   %MATLAB SCRIPT
   clearvars; 
   rng(1);

%1. PRELIMINARIES
% Add a path to the HMSC directory
addpath ('U:\MATLAB\HMSC-Matlab\HMSC class2');

% Assign the folder containing the simulated data
  dataFolder = 'U:\MATLAB\HMSC-Matlab\data2\simulated3';

    % Assign the folder where the results for this model will be saved (under
  % the results subfolder to be generated by HMSC)
  folder = 'U:\MATLAB\resultados2';

%2.READING IN THE DATA

file1 = fullfile(dataFolder, 'presabs1_Copy.csv');
Yda1 = importdata(file1);
Y = Yda1.data;
species1 = Yda1.textdata;

[Rik]

The problem occurs in the final part: importdata should load you csv file to a struct with one or more of these fields: data, textdata, colheaders, rowheaders. I would recommend using a more low-level approach and load the file with dlmread or textscan instead, and apply the necessary post-processing yourself.