You need to correct your syntax to:
=COUNTIFS('Tab1'!C2:C9,"sam", 'Tab1'!B2:B9,">1-Nov-2020")
Please read more on how the COUNTIFS function work.
EDIT (following OP’s comment)
The correct syntax would be
COUNTIFS(criteria_range1, criterion1, [criteria_range2, …], [criterion2, …]) meaning:
=COUNTIFS('Tab1'!C2:C9,"sam", 'Tab1'!B2:B9,">1-Nov-2020", 'Tab1'!B2:B9,">=1-11-2020")
BUT
Since you refer to dates 1-Nov-2020 is the same as 1-11-2020.
So you only need
=COUNTIFS('Tab1'!C2:C9,"sam", 'Tab1'!B2:B9,">=1-11-2020")
OR
=COUNTIFS('Tab1'!C2:C9,"sam", 'Tab1'!B2:B9,">=1-Nov-2020")
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