sp_textscan should be a 1x2 cell array, the first element, a cell array of strings (your dates) and the second a vector of numbers. So the correct syntax for datenum would be:
sp_formatted = datenum(sp_textscan{1}, 'mm/dd/yyyy');
However, assuming 2014b or latter, even better would be to tell textscan to parse the first part as a date straightaway:
sp_textscan = textscan(sp, '%{MM/dd/yyyy}D %f', 'HeaderLines', 1, 'Delimiter', ','); %note the different format string for datetime
