If some of your Instagram accounts aren’t listed on the Add Account pop-up, it most likely means that it doesn’t meet the requirements to be managed by a third-party App like Postcron.
First, make sure the account you’re looking to connect to Postcron is a Business account, and that it’s connected to a Facebook page, as the connection to Instagram is made through Facebook.
Also, notice that Postcron will only list Instagram accounts linked to the Facebook profile you’re currently logged into. So make sure you log into the right Facebook profile (the one that manages the Facebook page connected to your Instagram account).
Finally, make sure you have granted all the permissions needed for Postcron to post on your behalf. If you have denied or removed said permissions, you can accept them on your Facebook settings and allow Postcron to connect to the associated Instagram accounts.
Go to Business Integrations on your Facebook settings, and you’ll see Postcron listed as an Active Business Integration. Then, click on View and edit to edit the permissions.
Under Show a list of the Pages you manage you’ll see a list of the Pages that you’ve authorized Postcron to access with a checkmark and Pages that you haven’t authorized unchecked. If you don't see your Pages listed as shown below, it means that all your Pages are authorized.
Now, make sure that you check the Pages you'd like to add to Postcron (we recommend checking all of them to avoid having to edit your permissions again in the future), and don’t forget to Save the changes!