Tax problem - Please help!

If you really can't get to the tax office, ring them or write to them and tell them you have paid tax through your employer.
Have you asked your employer? It is not unknown for employers to deduct it but not pay it to the Tax authorities.
Do you receive a payslip showing the deductions? If you have not spoken to your employer, do so and tell them you will be sending copies of your payslip to the tax office to show your employer has been deducting tax. The tax office can then check whether your employer has actually passed it on to them and if not then I would imagine they would be very quick to claim it from them and possibly prosecute them as well.
It is also possible of course that the employer has miscalculated you tax liability and there is an outstanding amount.

Ray
( These are only my thoughts on the matter, I am not a tax advisor! )

If you started work last year then it may be that you are paying, via your employer, this years tax bill.
The tax demand may be tax for last year , maybe under estimate?
Check the dates on the tax demand.

Terry