HI, I FOUND THIS

COUNTIFSCounts the number of times each cells in all the ranges that meet the specific conditions.

COUNTIFS(test_range1; condition1;[test_range2;condition2];...)

test_range1  is the range to be tested.
condition  may be:
A number, such as  34.5
An expression, such as  2/3  or  SQRT(B5)
A text string
test_range2,condition2,… Optional. Up to 127 range/criteria pairs are allowed.
COUNTIFS  counts those cells in   test_range's  that are equal to the corresponding   conditions, unless  conditions  are text string that starts with a comparator's:
>,  <,  >=,  <=,  =,  <>
In this case  COUNTIFS  compares those cells in  test_range’s with the remainder of the text string (interpreted as a number if possible or text otherwise).

THEN I TRY THIS

=CUNTIFS( OBSERVACIONES.A2:A1000 ;>= A2 ; OBSERVACIONES.A2:A100 ;<= B2 )

IN OBSERVACIONES A2:A1000 I HAVE THE DAY THE CALL WAS DONE AND IN THE CURRENT SHEET I HAVE AUG / 01 / 2018 (A2) WHEN I WANT TO STAR MY COUNT AND IN (B2) I HAVE AUG / 31 / 2018

MISSION: TO KNOW HOW MANy CALLS I RECEIVED BETWEEN AUG 1st AND AUGUST THE 31st

WHEN I WROTE THE FORMULA, IN RETURN I GOT THIS!! 

#NAME!

WHAT DID I DID WRONG?