This is an example using a built-in dataframe
head(iris)
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> 1 5.1 3.5 1.4 0.2 setosa
#> 2 4.9 3.0 1.4 0.2 setosa
#> 3 4.7 3.2 1.3 0.2 setosa
#> 4 4.6 3.1 1.5 0.2 setosa
#> 5 5.0 3.6 1.4 0.2 setosa
#> 6 5.4 3.9 1.7 0.4 setosa
diag(iris[1,1:4])
#> [,1] [,2] [,3] [,4]
#> [1,] 5.1 0.0 0.0 0.0
#> [2,] 0.0 3.5 0.0 0.0
#> [3,] 0.0 0.0 1.4 0.0
#> [4,] 0.0 0.0 0.0 0.2
Created on 2019-03-04 by the reprex package (v0.2.1)
Assuming that you have imported your data correctly this should work
diag(USEEIOv1_1_Matrices_B[265,2:389])
In order for us been able to give you better help, please follow @Yarnabrina advice and make a reproducible example.