Apple has changed code signing behaviour starting Xcode 12.5 & above. Hence, one may get errors such as unsupported code signature while installing enterprise signed app on BrowserStack iOS 15 devices if it is code-signed with old Mac OS versions such as Mojave or Catalina.
For more details on the issue, please refer to Not able to install enterprise build app & The code signature version is no longer supported.
To solve this issue, ensure your enterprise signed iOS app is code-signed on MacOS Big Sur or above or use any of the workarounds mentioned in the provided links.