iPhone 8 Plus doesn't support iOS 16.7.13 software version

iPhone 8 and 8 Plus support the full version of iOS 16, but Apple Removed 16.7.13 due to reported problems whihc made Apple pull the update back. I believe that 16.7.14 might come soon, and you can update to that.

https://www.google.com/search?q=Does+iPhone+8%2B+support+iOS+16.7.13&sca_esv=95c4599e20cb2333&rlz=1CASFKO_enUS1181&ei=3tB8aZmhDNz8ptQP8qiTiAs&ved=0ahUKEwiZiMrQzLOSAxVcvokEHXLUBLEQ4dUDCBE&uact=5&oq=Does+iPhone+8%2B+support+iOS+16.7.13&gs_lp=Egxnd3Mtd2l6LXNlcnAiIkRvZXMgaVBob25lIDgrIHN1cHBvcnQgaU9TIDE2LjcuMTMyBRAhGKABMgUQIRigATIFECEYoAEyBRAhGKABMgUQIRigAUieXlCCWFiCWHACeAGQAQCYAZEBoAGRAaoBAzAuMbgBA8gBAPgBAvgBAZgCA6ACxgHCAgoQABiwAxjWBBhHmAMAiAYBkAYIkgcDMi4xoAfFBLIHAzAuMbgHogHCBwcyLTEuMS4xyAcmgAgA&sclient=gws-wiz-serp&safe=active&ssui=on

If you have downloaded the iOS 16.7.13 version to your iPhone, you will need to delete that file, so the new file can be downloaded when it is ready......probably sometime mid next week.

The release of this version has been halted by Apple. Therefore you cannot install it at this time.