OCR Chemistry A

how come relative atomic mass don't have a unit? can't it be 'u'?
Also does relative isotopic mass, relative molecular mass and relative formula mass not a unit, too?

No, because they're relative, not absolute.

The relative atomic mass etc. is just a ratio between the average mass of all the atoms in an element/molecule/whatever, and the unified mass unit. Both of these values have a unit of 'u', so 'u' divided by 'u' gives no unit.

Don't forget that the relative atomic mass etc. is relative to $\frac{1}{12}$ the mass of carbon-12 - this is what the unified mass unit is.