Why are the 'Tax Deducted' boxes on a Self Assessment return locked?
Before HMRC brought in new forms, you could enter the actual amounts for net, tax and gross income in boxes on your Self Assessment return. HMRC then used the actual tax deducted within the calculations.
These boxes are now locked (and there's no way of unlocking them within the software) because HMRC performs the relevant calculations on the figure entered in the net income box.
Note: If you've clicked on the wizard (the wand icon) to provide a further breakdown of the income, adjusting the tax deducted figure won't make any difference because HMRC just receives the total figure and calculates the tax from that.