Error -61 The TEXT document cannot be written when writing to a text file with Send packet

The documentation for Error -61 reads:
Read/write permission doesn't allow writing.

On Mac OS X here are couple of things to check for:

Under Mac OSX permissions play a critical role. Make sure 4D has permissions to both read and write to the file in which you're trying to write to with Send Packet.

Although 4D can read this file, hence it is able to open the document; 4D may not have permissions to write to this file.

In addition, make sure you aren't trying to write to a file which is already opened by another application.