This error occurs when a mandatory parameter for the action is missing.
Give the mandatory parameters when necessary.
Example
[#Form: Balance Sheet]
Add: Button: MyButton
[Button: MyButton]
Action: Display
In the above example, the mandatory parameter is missing for the action Display. Hence the message error T0042: Action 'Display' - mandatory parameter 1 is missing.
The code is corrected by giving the report name.
[#Form: Balance Sheet]
Add: Button: MyButton
[Button: MyButton]
Action: Display: Day Book