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 connected to their Aspire 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 has that network connected to their Aspire account. If they do not have that network, you can always ask them to connect it so you can propose terms.
If your creator is having trouble connecting their social account - please ask them to reach out to [email protected] or use the chat feature to request support.