You are trying to extract all the data to an array. In an array, all the data must be of the same type. The error is because your first column is doubles and your second is char. This is because readtable automatically detected the "521*" in row 34 of column2 and has set the datatype to be char.
If your goal is to have the data be in a matrix, I would suggest using readmatrix. You set up the read options appropriately to read in the desired range and fill the missing values with 9999. By making it 31x12, I assume you want to remove the first column? Note that the output here is missing the 
multiplier on the matrix values.
multiplier on the matrix values.opts = detectImportOptions("Arrone-pluviometro(36045)-2019-0-24.csv","Range","B1");
opts = setvartype(opts,"double");
opts.MissingRule = 'fill';
opts = setvaropts(opts,'FillValue',9999);
opts.DataLines = [2 32];
p = readmatrix('Arrone-pluviometro(36045)-2019-0-24.csv',opts)
However, my preference would be to keep your data as a table. You can use this page to learn how to access data in a table.
pT = readtable('Arrone-pluviometro(36045)-2019-0-24.csv',opts)
