If you plan to file your Resident tax return for income year 2020 (tax year 2021) yourself, you should have normally done so already before 30 June (on paper) or 20 July (online via Tax-on-web).
In case you missed the deadline, you can only submit your tax return for your 2020 earnings on time if you contact an accountant or tax advisor to file your return. The deadline in that case is 21 October, so you still have a couple of weeks left.
After that, you can be fined for not filing or for a late submission of your tax return. A tax increase can also be applied. Another risk of filing after the deadline, is that the taxman might issue a tax assessment ex officio based on the information that is readily available to them. This will shift the burden of proof to the taxpayer if the info included in the assessment would not be correct.
If you benefit from the special expat tax status or are considered a Non-Resident taxpayer, your tax return for income year 2020 needs to be filed by 4 November (on paper) or 2 December (online via Tax-on-web), so you have a bit more time left to file. The Non-Resident tax form has been available online since 15 September.
Feel free to reach out in case you would need any assistance with a (late) filing.