MLE, it's not clear what you have done, and what you want to do.
splitapply on a table is for cases when you want to apply a function that takes one input for each variable in a table. You may be seeng an error because you want to apply some function to each variable separately. You can use varfun to do that, or even easier, groupsummary, grouptransform, or groupfilter. Some of the latter three are fairly new.
>> t = readtable('data.csv');
>> groupsummary(t,'Group_ID',@mean)
ans =
25×5 table
Group_ID GroupCount fun1_E_0000_WH fun1_E_0030_WH fun1_E_0100_WH ...
________ __________ ______________ ______________ ______________ ...
1098 730 75.579 61.59 49.756 ...
1151 719 341.55 326.87 298.11 ...
1569 547 139.06 127.13 119.25 ...
1858 1356 160.29 136.18 118.33 ...
[snip]
