Proloquo2Go
You can share Proloquo2Go with the other Apple Accounts linked to your Family Sharing group. The app itself will remain officially linked to the account used to purchase it. For our apps that are one-time purchases, like Proloquo2Go, you will also need to enable Purchase Sharing. Once you have set up Family Sharing and Purchase Sharing, you can learn more here: How do I share Proloquo2Go using Apple's Family Sharing?
Having too little available storage on the device can cause this issue. For details on how to check your storage, please go to the following Apple Support Article: https://support.apple.com/en-us/108429. Delete any unneeded apps, videos, etc. It's important to have at least 5GB of available storage. Once you have freed up space, please close Proloquo2Go and force restart your device.
You can redownload purchased apps for free. Please be sure to sign in with the Apple Account / ID that you used to purchase or download Proloquo2Go originally. You can find instructions in the Redownload an App section of https://support.apple.com/en-us/102417. When you re-download an app from the App Store there are no customizations. So you will want to save a Proloquo2Go backup file from the original device to transfer to the new device. For instructions on saving and then sharing your Proloquo2Go vocabulary, please see: Protect, recover, and share vocabulary (https://www.assistiveware.com/support/proloquo2go/protect-share).
To keep Proloquo2Go user vocabulary in sync across multiple devices, save a backup file of the vocabulary you want on all the devices and then transfer it to the others. For instructions, see the Protect, recover, and share vocabulary section on our website. (https://www.assistiveware.com/support/proloquo2go/protect-recover-and-share-vocabulary/).