With the absolute lack of details, I can only conjecture. Maybe your browser is Chromium-based, or you're using uBlock instead of uBlock Origin, or you have viruses, or you disabled it without noticing, or you managed to whitelist every domain (is that even possible though), etc.

If you're using browser to use youtube, you can try invidious, an alternative frontend.

redirect.invidious.io

sometimes I have to manually update my filter lists, but also ensure you're using ublock origin (as already stated in a previous reply itt)

I had to reinstall my browser, some settings were bugging it our after youtube updated something

I switched to uBlock Origin Lite since around that time and it has been blocking youtube ads just fine.