Conditional enable/disable of subform rows

My app contains 2 forms:

MainForm = Contains some data in various fields
Subform = contains comments in the form of multiple rows

My users can add new comments by adding rows. All this is working fine. What is needed is:

1. Disable all rows "comment field" when the MainForm is edited e.g. "disable comment;"
2. On Add Row, enable the field e.g. "enable row.comment;"

I can successful achieve #1 by adding "disable comment;" in the Edit -> On Load. However, when I add "enable row.comment;" on the "On Add Row" I am getting the following error:

Error at line 5 : This task is not allowed inside subform's workflow

Anyone have an idea on how to achieve this or what this error means? Ultimately, I also wish to control delete making all the comments permanent with no edit or delete rights.

Thx