When can I write off an expense, year accrued or year paid?

I had work done on my commercial property during 2019 that required putting up a scaffolding during the duration of the repairs. They are now complete and I received the final bill for the payment of the scaffolding. 

Do I write off this scaffolding expenses as part of 2019 when the work was being completed or in my 2020 tax return because I will only be paying it now?