How to resolve formula discrepancy Excel/SharePoint

As already mentioned by Kidd_Ip, the problem may (or is likely) the syntax for the online uses semi-colons instead of commas.  That said, can I comment on the formula itself:

=IF(  ISERROR(INDEX(Form1!F:F, ROW()-90)), 
         "", 
         IF(  INDEX(Form1!F:F, ROW()-90) = "", 
               "", 
               INDEX(Form1!F:F, ROW()-90)
              )
        ) 

so the 1st part checks if the INDEX is an error and print "" if so

2nd part checks if INDEX returns "" and print "" if so

3rd part returns the answer from INDEX

so why even do the 2nd part because if it will return "" anyway then just skip to part 3

as for the ISERROR why not just use IFERROR so it could look like this:

=IFERROR( INDEX(Form1!F:F, ROW()-90), "") 

and that said, I'm not a fan of using hardcoded values and that 90 could be a problem.  For example you or someone else a year or 2 from now decide the first 10 rows aren't needed and you just Delete those rows.  Then all the data shift up by 10 rows and now instead of 90 it should be 80.  So nothing is perfect but I would suggest an option might be to use the cell $F$90 because when those rows are deleted that cell $F$90 will automatically be adjusted to $F$80 so something like this:

 =IFERROR( INDEX(Form1!F:F, ROW()-ROW($F$90) ), "")  

note the column F doesn't matter but the '$' does so if you copy or drag down that cell reference doesn't change.