Hello !
Sometimes, on the classic Outlook, the attachment icon can be hidden if your Outlook window is too small. Make sure it's fully expanded.
The attachment tools are also in the "Insert" tab of the ribbon : can you see them ?
If you cannot see these tools anyway, you can reset the Ribbon layout by clicking on File > Options > Customize ribbon > Reset > Reset all customisations.
Please let me know if you have any other question !