Struggling with errors in relationships

Hello,

I'm trying to set up a 1:Many relationship from one table which is a list of unique employee IDs (no duplicates or empty) to another table which has lots of occurences of the employee ID. I try selecting the 1:Many relationship with my unique list at the top, but it says the cardinality you selected isnt valid. This doesnt many much sense as the data is unique and cleaned up. It will only accept Many:Many which I dont want to use. 

As a check, I tried removing duplicates in the unique table which didnt remove any rows. I then closed the transform view and applied changes and get the error "ole db or odbc error dataformat.error we couldn't convert to number" which makes no sense as the columns are formated as text, integers or decimals. No columns are formatted as dates or contain dates. 

Any suggestions?