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_IDGroupCountfun1_E_0000_WHfun1_E_0030_WHfun1_E_0100_WH...
____________________________________________________________...
109873075.57961.5949.756...
1151719341.55326.87298.11...
1569547139.06127.13119.25...
18581356160.29136.18118.33...
[snip]
