Depending on the payment platform you have chosen to pay for your Flightradar24 subscription you can find your receipt for your payment as follows:
Payment online via www.flightradar24.com:
Please follow this link: https://www.flightradar24.com/account/billing. At the bottom of the page, you should see the section "Billing History". In order to download a copy of your receipt please click on the blue invoice link visible next to the date. 
Payment via an invoice sent to you by our B2B team:
In this case, you have received an invoice to pay there is no receipt for your payment sent. Proof of payment is displayed as a deduction on the bank account.
Payment via Apple:
Please follow this link for more information: https://support.apple.com/en-gb/HT201382?cid=appledotcom_Billing_article_HT201382
Payment via Google:
Please follow this link for more information: https://support.google.com/store/answer/13037491?hl=en&visit_id=638121515665204616-2412500131&rd=1