Hi @justlea,
I think you're close with how you have it set up, assuming you have a numeric variable in your Gruppe_LM dataframe.
The error is stating that the variable that you have given the hist() function is not numeric. So R is reading the Gruppe_LM$Bilder variable as something other than a numeric.
I believe the problem here is within your subset function, as I believe you are missing the subset expression, which should be listed after the dataframe Gruppe_LM. This should define some criteria of values to keep. For example let's say your Gruppe_LM dataframe has a variable called value, and it has a value from 1 to 20. But you only want to subset your data where the value is greater than 10.
Gruppe_LM$Bilder <- subset(Gruppe_LM, value > 10, select = c(001, T002, T003, T004, T005, T006, T007, T008, T009, T010, T011, T012, T013, T014, T015, T016, T017, T018, T019))
That should get your subset function working, however the other piece of this I'm curious about is your Gruppe_LM$Bilder variable. Are you passing those 19 columns (T001 through T019) into one column called Bilder?
An example of your dataframe would be helpful to troubleshoot this further. Here's a great post by user milesmcbain with info from EconomiCurtis and jessemaegan.
If you could please include a small example of your dataframe, this would help illustrate what data you are trying to get to show up in your histogram.
Cheers!