Strictly speaking, an employee can’t be compelled to resign. A resignation must be the voluntary act of the employee.
For an employee’s actions to amount to a resignation, they must communicate or evince a clear and unequivocal intention to resign. If an apparent resignation was not voluntary, a court could rule that the employee was dismissed without cause or notice and award damages.
Sometimes an employer will ask an employee to tender their resignation when it thinks there is cause for termination. Employees may feel inclined to comply to avoid the stigma of a cause allegation, but they should be very careful before taking that step. There may not actually be just cause for termination, but a resignation would generally disentitle the employee to pay in lieu of notice.