JavaScript

/^|$|\t|\s{2,}/gm

/^|$|\t|\s+/gm

1. You've spelt "split" as "slpit" which will cause an error.
2. The split() method returns an array of string objects, not a simple string, so your msgBox needs to reference one of those - in your case myvar[0]

Browser.msgBox('Welcome to my spreadsheet '+myvar[0], Browser.Buttons.OK);

function onOpen() {
  
var MyName = Session.getActiveUser()
// Browser.msgBox(email)
var myvar = MyName.split("@");
   Browser.msgBox('Welcome to my spreadsheet '+myvar[0], Browser.Buttons.OK);
  
}

Session.getActiveUser()

var MyName = Session.getActiveUser().getEmail()

Member 12269518 wrote:

but I still receive an error . . .

var array = [
   ['thing1','stuff1'],
   ['thing2','stuff2'],
   ['thing3','stuff3'],
]

array[0] === ['thing1','stuff1'];
array[0][0] == 'thing1';
array[0][1] == 'stuff1';

array [2][1] == 'stuff3';

array[0][0][0] === 't';
array[0][0][1] === 'h';
array[0][0][2] === 'i';
array[0][0][3] === 'n';
array[0][0][4] === 'g';
array[0][0][5] === '1';