The HMRC guidance document confirms the below have to be applicable for the relief to be available:

Individuals will be able to claim a basic rate tax reduction from their Income Tax liability on the portion of finance costs not deducted in calculating the profit. In practice this tax reduction will be calculated as 20% of the lower of the:

• finance costs not deducted from income in the tax year (25% for 2017 to 2018, 50% for 2018 to 2019, 75% for 2019 to 2020 and 100% thereafter)
• profits of the property business in the tax year
• total income (excluding savings income and dividend income) that exceeds the personal allowance (PA) and blind person’s allowance (BPA) in the tax year

Therefore if there is no property profit, you will not be eligible for the relief OR if the total income (excluding savings income and dividend income) is lower than the PA and BPA in the year, this would not be available either.

Outside of this, if relief is being applied to the return, you may need to look at the HMRC restriction calculation as below:

HMRC restrict the relief to the lower of the ‘Adjusted Total Income’ and the ‘Allowable Amount’. The allowable amount being the amount entered in box 44 on the tax return. The calculation that HMRC apply is Adjusted Total Income is calculated as Net Income - (Savings income + Dividend income + Allowances). Adjusted income can be found in box A115 on the HMRC tax calculation summary in Section 4.