Populate data in lookup field via upload

Structure:
App 1, (AR) contains 4 tables, the working table(Invoices), and three lookup tables from App 2 which contain the tables: Client, Program and Sites (contains 1000's of sites).  I need to populate the Site Name on the Invoices table from the Sites list.
 
I have a new app that I created from a spreadsheet, where information wasn't entered in correctly or completely. So I have added a lookup field from another table (which actually feeds from another app) where the information is uniform. I am now trying to populate the value of that field on the parent table. I don't want to have to go through each line item and edit the field to pull in the information. I exported the "sites" table so I could have exact match to put in via upload, but because it's a lookup field, I can't choose to import to that field on the Invoices table. I have tried to copy and paste from one field into the lookup field, but it won't populate either.
I can't figure out why the field won't show the data even though it is being entered exactly as it appears on the Sites table. 

Is there another method I can try? this is a one time thing. Going forward the original field will be deleted and the user will only be able to use the lookup fields.