You didn't specify but I'm going with the assumption that payroll is biweekly: 1/11, 1/25, 2/1, 2/15, 3/1, 3/15, 3/29, 4/12, 4/26, 5/10, ...
(A) Failure first occurred: 1/11/2019
(B) Last day of the three-month that begins on (A): 4/11/2019
(C) First payment of compensation made on or after (B) 4/12/2019
(D) Plan sponsor notified of the failure by affected eligible employee: 3/27/2019
(E) End of month after the month of (D): 4/30/2019
(F) First payment of compensation made on or after (E): 5/10/2019
Earlier of (C) and (F) : 4/12/2019
Correct deferrals need to begin no later than 4/12/2019.