In your situation, you can have 2 options to choose:
1. If you have business expenses when you haven’t yet started earning income, you can fill as a self-employed and claim the business expense providing that you are actually generating those expenses to earn income. The expenses must be reasonable, and you must have receipts & logs to back up the amounts you claim (should the CRA ask). It also helps if you have other documentation to show that your business is real. A business plan, advertising you had printed up, etc. - things that show that you are genuinely going into business, even if you haven't earned anything yet.
Please check our TurboTax articles for more details:
When Can I Claim Business Expenses?
Deductible Business Start-Up Costs
You can file a nil return for 2020 in business income and then you should not send it in again. If you do not intend to return to self-employment, you can ask to close your tax account at CRA.
2. If you have no business expenses as well. You do not have to state that you are self-employed if you have no business income to report.
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