When trying to fulfill from transfer order on 'create' i need to get the inventory detail rows. when i do:

let invDet = rec.getSublistSubrecord(sublist, 'inventorydetail', i);
let invDetCount = invDet.getLineCount('inventoryassignment');

i always get 0 of invDetCount. (I have a serial there for sure, and the number of rows in 'item' sublist is 1) the script is on afterSubmit

Are you loading the record on afterSubmit?

No. (If i do its works...) But its not strange that on afterSubmit I cant get the inventory detail ?

You need to load the record on afterSubmit, otherwise you are not going to have access to all the information the record may have.