Somewhere in the code swap the high byte and the low byte. You don't show the calling sequence and your default bit width is 24, so I'm not sure how to code this generally, the requirement is not clear. So I'll assume the requirement is to swap the high bits with the low bits regardless of bus size.
out_data(VERI_UZUNLUGU -1 downto VERI_UZUNLUGU / 2) <= r_data((VERI_UZUNLUGU / 2) -1 downto 0);
out_data((VERI_UZUNLUGU / 2) -1 downto 0) <= r_data(VERI_UZUNLUGU -1 downto VERI_UZUNLUGU / 2);
VERI_UZUNLUGU must be even. You might want to add an assert statement to verify this. But not required since an odd value will cause a mismatch in these assignments and cause an error. An assert will give a very clear error report though instead of a cryptic syntax error.
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