Read_text is used for reading long text created in SE75, not for text stored in table.
If you want to create a long text, dont store it on table level
Instead go to SE75, go in change mode, create a new text object, Create Text ID, specify length and key (it will be key of your QPCD table), you can create one dictioanry structure containing key fields and specify here).
Once it is created. You can save text here using SAVE_TEXT function module, and read it via READ_TEXT function module.
Passing text object, text ID and key created above in SE75