There are several reasons. Maybe your account has some restrictions, or the IG platform is undergoing maintenance. Another possibility is that you haven't updated the app to the latest version.
Maybe your internet connection is unstable. Check if your Wi-Fi or data is working properly.
It could be that your IG app is outdated. Try updating it to the latest version and see if that helps.
Maybe there's a glitch in the app. Try restarting your device or reinstalling the Instagram app.
Maybe there's a problem with your internet connection. Check if it's stable.
Sometimes, it might be because of a glitch in the Instagram system. Wait for a while and see if the issue resolves itself. Also, check if you have enough storage space on your device for the upload.
Maybe there's a glitch in the app. Try restarting your device or reinstalling the Instagram app.
Maybe there's a technical glitch or a problem with your internet connection. Check those first.
Could be that the original post has settings that don't allow reposting. Or your account might have some restrictions due to previous violations of IG's rules. Have a look at IG's help section for more info.
It could be that your IG app needs an update. Try updating it and see if that helps. Also, make sure you haven't violated any of IG's posting rules.
Maybe there's a glitch in the app or your internet connection is unstable.