Online orders fail on certain websites since December 23

Even if you aren't using a VPN on your phone, but one is there, it can cause issues like you're seeing. Don't just turn off the VPN. GET RID OF IT. Open Settings > General > VPN & Device Management - remove the VPN Profile. Then if there is any associated app for the VPN, delete it. Once done, Force Restart your iPhone --> Force restart iPhone - Apple Support

VPN is simply not needed on iPhone. They cause more problems than they could solve and don't in any way provide the security people have mistakenly been led to believe. Read this about using a VPN on iPhone --> VPN: What you need to know - Apple Community

That sounds like an issue with those sites, not your iPhone. You've verified you can place orders on some sites. That rules out the phone. Contact the site if you can't successfully place an order with them.

KatherineElaine wrote:

I checked my VPN and it has been turned off for a while. I contacted two of the stores I shop most frequently with no follow up from either. I went through everything on my phone to see if anything had been changed. I have deleted apps and reinstalled. Rebooted. I have run diagnostics and virus scans.

My brain is exhausted.

Remove the VPN entirely. also remove whatever 3rd party antivirus you've installed. Neither one will accomplish anything other than causing problems. They are doing nothing to protect you.

If you have any add-on VPN apps installed, remove all, restart, and try again.