When an exchange straddles years (assuming you had Intent to complete the exchange) you can choose to take in the current year or treat as an installment sale and recognize in the following year. Bear in mind you may still have tax on depreciation recapture in the year of the exchange. Here are some articles which may help. You can find many more by searching "Failed 1031 tax straddles".
https://www.firstexchange.com/content1031-exchanges-straddle-two-tax-years
https://www.ipx1031.com/taxstraddling/
https://www.eisneramper.com/1031-exchange-1119/
https://www.accruit.com/blog/your-1031-exchange-straddling-two-tax-years
https://www.ipx1031.com/will-pay-taxes-2017-2018/
https://www.ipx1031.com/impact-of-depreciation-recapture-on-exchanges/
Hope this helps. It is actually difficult once you get your head around the options.
