Calculation error in measure

Maybe just a copy/paste error, but your calculation is missing a closing parentheses.  What is the data type of Personnel_Records[Resignation]?  If it's not a string then there's no need to enclose the 1 in double quotes.  You can also try passing the date to the [Leaving Date] explicitly as a date.

CALCULATE(
	COUNT(Personnel_Records[Resignation]),
	FILTER(
		'Personnel_Records',
		'Personnel_Records'[Resignation] = 1 && 
		'Personnel_Records'[Leaving Date] > DATE(2021, 1, 1)
	)
)