Both MyTaxExpress/T2Express have a "Validate" feature to help you detect some common errors in your return. It is recommended that you run validation before filing your returns. If the return can't pass the validation, the software will not proceed to do any electronic filing.
To start the validation process, simply press the icon "Validate" at the top of the software window. An Error window will pop up to display errors detected. You can correct those errors accordingly, then repeat this process until no errors are found.