You could have filed an extension (Form 7004), but it needs to be timely filed and the return was due by the 15th day of the 3rd month following the date the LLC's tax year ended.

See https://www.irs.gov/instructions/i1065#idm140538038386032

You can use the 2018 Form 1065 provided the 2019 Form 1065 is not available by the time the LLC is required to file its return (which it is not) and you incorporate any tax law changes that are effective for tax years beginning after 2018.

See https://www.irs.gov/instructions/i1065#idm140538038374016