Troubleshooting: Cannot select tax code for past document

Question: Why can't I select tax code for past document?

Version: 1.8 / 1.9 / 2.0

Possible Reason:

The GST logic is set to effective only after GST Start Date.

Note**:The first option: ‘Apply Tax Logic only after GST Start Date’ will cause all GST functions to work only on/after the GST Start Date; while the second option: ‘Apply Tax Logic for all date’ means all GST functions will be effective as soon as GST is enabled.

Solution:

Go to Tools > Options > General > Goods and Services Tax > change to Apply Tax Logic for all date