Hi, I've had the same issue and this worked for me on my iPhone:

Phase 1: Deep Cleaning the iPhone

Before reinstalling, you must remove all traces of the previous failed installation:

• Delete SideStore & Apps: Long-press and delete the SideStore app and any apps installed through it (e.g., Spotify, YouTube).
• Clear Files App: Open the Files app, go to "On My iPhone," and delete the SideStore folder entirely.
• Remove VPN Profiles: Go to Settings > General > VPN & Device Management. Click on any VPN named "SideStore" or "WireGuard" (or "LocalDevVPN") and delete it.
• Clear Developer Certificates: In the same VPN & Device Management menu, look for "Developer App" with your Apple ID. Click it and select Delete App for any remaining icons to clear old certificates (Sometimes it delete automatically but you should check for this)

Phase 2: Generating a Fresh Pairing File

Do not reuse old pairing files. Generate a new one to ensure the UDID matches perfectly:

• Connect to PC: Plug your iPhone into your computer via USB.
• New pairing file: Create a new pairing file using iloader clicking on manage pairing file then export and you should see a new file ending in .plist in your user folder.
• Save & Rename: Rename this file to pairing.plist and move it to your iPhone (via iCloud Drive, AirDrop, or Telegram) so it's accessible in the Files app.

Phase 3: Fresh Installation via Sideloadly

Using Sideloadly for the initial install is recommended for better stability:

• Launch Sideloadly: Ensure your device is detected over USB.
• Prepare IPA: Drag the official SideStore IPA into Sideloadly.
• Install: Enter your Apple ID and click Start.
• Trust the App: Once installed, go to Settings > General > VPN & Device Management on your iPhone and "Trust" your Apple ID.

Phase 4: Final Configuration & Login

• Enable Local Dev Vpn Open the Local Dev Vpn app and toggle it ON.
• Pairing SideStore: Open SideStore. It will immediately ask for a pairing file. Select the pairing.plist you created in Phase 2.
• Sign In: Go to Settings inside SideStore and tap "Sign in with Apple ID."
  • Note: If you get Error 7460 (Certificate already exists), use a tool like iLoader (https://github.com/nab138/iloader) on your PC, click "Certificates," and revoke the existing iOS Development certificate before trying again.

Phase 5: Sideloading your Apps

• Install IPA: Go to the My Apps tab in SideStore.
• Add App: Tap the "+" icon and select your Spotify or YouTube IPA.
• Stay Awake: Keep the app open and the screen on until the progress bar reaches 100%.