This formula should look up in the A column of multiple sheets and when the match is found (in this case there are not duplicates in any A column of the different sheets), it gives back the value found in the cell next on the right to the match.
=ArrayFormula(IF(LEN(B5),VLOOKUP(B5,{SHEET1!$A$3:$B,SHEET2!$A$15:$B},2,FALSE),""))
But it gives a mismatch error and even if I make the ranges the same length the value in B5 is searched only into the first sheet of the range, in this case SHEET1!$A$3:$B.
Is is possible to make the formula work with ranges of different length from multiple sheets?