Also, check if you have a double certificate. This will now change your insecure SSL setting and the error will be fixed. Try downloading again, making sure you have a strong internet connection - only use the official sources for the IPA files too.
This is down to the original IPA file not being archived correctly, or the download was interrupted. Remove it completely and download it again from the official source only.
The easiest way to fix this is to delete iTunes and reinstall it on your computer - you must have the most up to date version for Cydia Impactor to work properly. If the error persists, disable two-factor authentication on your existing ID and try again. You can also try creating a new Apple ID and trying that. Make sure you have a firewall and a strong internet connection and, if you are using a VPN, turn it off - you can enable it again later. Delete it from your device and download it again, making sure it is the newest version. This is most likely down to you using an out of date version of Cydia Impactor. After the file has been installed on your device, you need to trust the developer. This is relating to the app certificate, and the error message will tell you that you already have a pending development certificate request. This is down to iTunes not being up to date.
The only way to resolve this is to upgrade to the latest iOS and then try again. There are a couple of things you can try. There is a way to resolve this, just follow these steps. This error message is warning you that there is a conflict between iOS development certificates used for sideloading IPA files. When you can see that the developer portal is up and running, you can go ahead and use Cydia Impactor to sideload your apps. This can be monitored by visiting developer. You can only stop this happening by using Cydia Impactor when the developer portal is running properly.
These two are slightly easier to fix and tend to appear when the Apple Developer Program goes down for maintenance. Cydia Impactor is now a different tool altogether, used for sideloading apps onto an iPhone or iPad.