I should have included this article link:

https://thelink.ascensus.com/articles/2020/4/15/plan-establishment-deadline-changed-by-secure-act

"If an employer establishes a plan for the prior year, can elective deferrals be contributed for that prior plan year?

No. Employees may not defer retroactively into a qualified retirement plan. The rules requiring that elective deferrals be made during a plan year—before being received, in fact—were unaffected by the SECURE Act; therefore, employee elective deferrals must still be contributed on a prospective basis only.

What types of contributions can be made for the prior year?

The plan sponsor may elect to make employer contributions to a defined contribution plan. Depending on the type of plan established, this could include profit sharing contributions or qualified non-elective contributions. If the newly-established plan is instead a pension plan, including a cash balance plan, the plan sponsor may contribute employer contributions to that pension plan. Note that these are all employer—not employee—contributions."