There are 2 main reasons you can't propose terms for a specific networks. Either your campaign is not set for this kind of content or the influencer doesn't have that network connect to their AspireIQ account.
You will want to confirm that the requested network is selected during campaign setup here:
Then within the Manage Tab you will also want to confirm that the Creator also has that network connect to their AspireIQ account. If they do not have that network connected you can always ask if they will connect it so you can propose terms.