why cant i attach mods to weapons?

ik you cant attach mods to iconic weapons but ive tried on some nnormal weapons nad can onyl attach attachments and not mods,why?

https://preview.redd.it/u5fhrcbdkqcg1.png?width=1679&format=png&auto=webp&s=783f6a6cfa6de05811a0b4160f2adb8c9c5f6011

https://preview.redd.it/repgmzbfkqcg1.png?width=1669&format=png&auto=webp&s=e995def9264b7898e91e4d058c15322417a67836

i searched it up and it said that mods would be available right under the attachments,but nothing is showing up in terms of mods,why?