When doing a split of participant variables, not all variables are selectable.
You only can access participant variables of type "selectbox" (integer values). Participant variables of type text or text field are not available because they are text and you can not e.g. calculate an average out of text. That also is the reason why you can not select them in recodings for the report.
At the moment your project apparently only has three variables of type selectbox: u_gender, u_group and u_language.
If you need variables of other types in the report, you have to change the variable type first. You directly can do this in participant administration >> participant variables.
This process should not include data loss of answer categories or result data (I already did this before and it never deleted my data). To be sure it though is suggested do an export of participant and result data first.
When changing the variable type in participant administration >> participant variables, the answer categories of all participants automatically are adapted and changed to a number/ID.
After this process you can find the variable in the reporting and e.g. use it as splitting dimension. Should there be duplicate answer categories, e.g. "Questback" and "Questback AG", you can recode the variable first so there only are distinct answer categories and then use the variable for a spit in the reporting.