Hi Carlos
Here is how to center-align text in a Document table cell:
1. You have to create a center aligned attribute style
2. Apply the style to each paragraph in the table cell
Here is a simple example that center-aligns the text in the first cell of the first table in the active document, hopefully this will help
Ed
function myFunction() {
var myTable = DocumentApp.getActiveDocument().getBody().getTables()[0]
var centerAlignStyle = {}
centerAlignStyle[DocumentApp.Attribute.HORIZONTAL_ALIGNMENT] = DocumentApp.HorizontalAlignment.CENTER
myTable.getCell(0,0).getChild(0).setAttributes(centerAlignStyle)
}