When to make election.

Generally, you must make the election on a timely filed tax return (including extensions) for the year in which you place the property in service.

However, if you timely filed your return for the year without making the election, you can still make the election by filing an amended return within 6 months of the due date of the original return (not including extensions). Attach the election statement to the amended return. On the amended return, write “Filed pursuant to section 301.9100-2.”

https://www.irs.gov/publications/p946#en_US_2020_publink1000299557