If you live in Sweden and lived here before the United Kingdom left the EU, you should have applied for residence status as a British citizen by the deadline for doing so, 31 December 2021. If you did not live in Sweden before 31 December 2020, you cannot be granted residence status as a British citizen.
Normally, it is therefore too late to apply for residence status as a British citizen, but in exceptional cases you can still apply for residence status, despite the fact that the application deadline has passed. In that case, you must submit a written explanation of why you have been prevented from applying on time together with your application. It is then the Swedish Migration Agency that assesses whether your explanation provides reasonable reasons for why you have applied too late. Not knowing the deadline for filing an application is not a reasonable (valid) reason.