test$ cat test.asm

section QUUX class=QUUX
        db 26h

section FOO align=BAR class=FOO
        align BAR
        nop

section STACK class=STACK stack align=16
        resb 512
test$ ~/proj/nasm/nasm -f obj test.asm -DBAR=8 -o test8.obj -Wall
test.asm:5: error: foo
test.asm:5: warning: OBJ format does not support alignment of 8: rounding up to 16 [-w+other]
test$ nasm -v
NASM version 2.16.02rc2 compiled on Oct 12 2023
test$ nasm -f obj test.asm -DBAR=8 -o test8.obj -Wall
test$