Hey Rajnish,
Since you are able to update the first field, I believe your BDC call is working fine. To better understand the error, I can recommend you try the following:
1. Try using the T. Code: SE38
2. Instead of passing the value through the field 'ZACTIVITY_SCREEN-TEXT', hard code the value to 'ABCD' to test the code.
3. For two lines of data, you will have to do some research as most of the screen elements have an output length limitation of 256 characters. This is where FM 'SAVE_TEXT' helps. But, first confirm the hard coded value is saved.
Regards,
Pranav.