Tax planning is only possible for the current fiscal year, say for in April 2021 one can plan taxes for FY21-22. This is because the income position for the current fiscal year may be predicted. Tax breaks and deductions can also be identified, and it may be determined whether they are being maximised. Based on that, one can plan their taxes. The ongoing fiscal year's quarterly advance tax liability can also be plotted and paid.
This is why you see the drop down on tax planning page in the tax planner app greyed out, since it enables financial and tax planning only for the ongoing financial year.