The reason was already explained to you in https://www.mathworks.com/matlabcentral/answers/340974-undefined-function-sum-for-input-arguments-of-type-struct-and-error-in-std#comment_455195
"If 01911m is a MAT-file, the syntax you've used for load will make x into a struct array with one field per variable in the MAT-file. You probably don't want to take the std of the struct but you want to operate on one of the fields of the struct instead"
You should look more closely at the documentation for load(): https://www.mathworks.com/help/matlab/ref/load.html#outputarg_S . When you assign the output of load() to a variable then the output is:
"Loaded variables, returned as a structure array, if filename is a MAT-File.
Loaded data, returned as an m-by-n array of type double, if filename is an ASCII file. m is equal to the number of lines in the file, and n is equal to the number of values on a line."
You are encountering the MAT version, so the output of load() is a structure array.
