Difference problem

I am very new in this world of power BI,
Here is my problem: I wanted to calculate the difference between two columns on each column in a different table, here is the formula I use   Ecart2 = ifp_vte1[Mt HT] - RealiseN[Mt HT],
And this reminds me of the following error:   It is impossible to determine a unique value for the 'Mt HT' column of the 'ifp_vte1' table. This can occur when a measurement formula refers to a column that contains many values, without specifying aggregation such as min, max, count, or sum to get a single result.