Could you ask this with a minimal REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.
You should look at the documentation about
lm to see how the formula interface works. It would better to put your two variables in a data.frame and use something like this
ModelBelgium <- lm(ShareBelgium ~ MVBelgium, data = my_data)
invalid type (list) for variable 'ShareBelgium'
Otherwise, be sure that
MVBelgium are vectors.
From the help page example:
ctl <- c(4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14)
trt <- c(4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69)
group <- gl(2, 10, 20, labels = c("Ctl","Trt"))
#>  Ctl Ctl Ctl Ctl Ctl Ctl Ctl Ctl Ctl Ctl Trt Trt Trt Trt Trt Trt Trt
#>  Trt Trt Trt
#> Levels: Ctl Trt
weight <- c(ctl, trt)
#>  4.17 5.58 5.18 6.11 4.50 4.61 5.17 4.53 5.33 5.14 4.81 4.17 4.41 3.59
#>  5.87 3.83 6.03 4.89 4.32 4.69
lm.D9 <- lm(weight ~ group)
#> lm(formula = weight ~ group)
#> (Intercept) groupTrt
#> 5.032 -0.371
Created on 2018-11-11 by the reprex package (v0.2.1)