Excel IFS function

I use Excel IFS function to add *

The working function is:

=IF(G5="","",FIXED(G5,1)&IFS(ABS(G6)>=2.575,"***",ABS(G6)>=1.96,"**",ABS(G6)>=1.645,"*",ABS(G6)<1.645,""))

Since I see ABS(G6)<1.645 is redundant so I decided to drop it

=IF(G5="","",FIXED(G5,1)&IFS(ABS(G6)>=2.575,"***",ABS(G6)>=1.96,"**",ABS(G6)>=1.645,"*"))

But the formula run into issue return #N/A

I hope someone can explain for me why removing part cause the issue?

You need to provide the "else" branch for IFS:

=IF(G5="",
  "",
  FIXED(G5,1)&IFS(
    ABS(G6)>=2.575,"***",
    ABS(G6)>=1.96,"**",
    ABS(G6)>=1.645,"*",
    TRUE,""
  )
)

Read Remarks in IFS function:

To specify a default result, enter TRUE for your final logical_test argument. If none of the other conditions are met, the corresponding value will be returned.

• If a logical_test argument is supplied without a corresponding value_if_true, this function shows a "You've entered too few arguments for this function" error message.
• If a logical_test argument is evaluated and resolves to a value other than TRUE or FALSE, this function returns a '#VALUE!' error.
• If no TRUE conditions are found, this function returns '#N/A' error.

So, you've got '#N/A' due to ABS(G6)<1.645 and there is no such condition in the list.

Also, you can suppress '#N/A' by another way:

=IF(G5="",
  "",
  FIXED(G5,1)&IFNA(
    IFS(
      ABS(G6)>=2.575,"***",
      ABS(G6)>=1.96,"**",
      ABS(G6)>=1.645,"*"
    ),
    ""
  )
)