In regards to the selector:
#tx_alterneteducationcatalog_subscriberadd[newSubscriber][gender]-error
It appears to be invalid due to the square brackets representing attribute selectors. This results in:
#tx_alterneteducationcatalog_subscriberadd
[newSubscriber]
[gender]
Following this, there is a syntax error with -error since an identifier does not belong in that position.
To ensure the correct parsing of the entire ID selector, all square brackets should be escaped like so:
#tx_alterneteducationcatalog_subscriberadd\[newSubscriber\]\[gender\]-error
An alternative approach would be using an attribute selector to target the ID:
[id="tx_alterneteducationcatalog_subscriberadd[newSubscriber][gender]-error"]
Another option could involve selecting by the for attribute and the .error class rather than solely relying on id. However, it seems unclear why a validation error would be associated with a label element without specifying the exact field being referenced; the indication simply states that the field is required.
Note: There appears to be a line break within your id attribute. If this matches the actual structure of your markup precisely, then it violates standard format guidelines. Nevertheless, you can accommodate this issue in your CSS by utilizing the escape sequence \a (refer to section 4.3.7 of the specification):
#tx_alterneteducationcatalog_subscriberadd\[newSubscriber\]\a\[gender\]-error
[id="tx_alterneteducationcatalog_subscriberadd[newSubscriber]\a[gender]-error"]