A drop-down option only occurs if there is only one code you can choose for the field. A drop-down wasn't used for the Literary Text Code because it is possible to have up to 2 codes. The default for the code is two blanks. Backspace over the default blanks and enter one or two codes to complete this field.
The Question Mark icon to the left can be used to bring up the Help for the Bibliographic Formats and Standards.