When I attempt to reply, there is no Attach option. I think it's because I just started using this forum and there is some minimum number of posts that I must make before the Attach option appears for me (sigh).
It should be there right from the start, clicking the Reply option at the bottom of the page (image1) or the Quote option in a post you wish to reply to it should be below the text window.
However, if you are using the Quick Reply option you may not see it as it just shows a text window, in which case, there should be a Preview button and that should give you the full Reply window including the Attachments and other options, image2. Clicking that text should expand that option.
What are you using when trying to Reply laptop/desktop or mobile phone ?