Can I buy activation code for steinberg cubase pro 10.5
I don’t think Steinberg sell licenses for older versions. You would have to buy from a private seller. You will also need a dongle to put the license on. You could try looking on eBay, KVR and Reverb for someone selling a 10.5 license with a dongle.
Be careful though as a dongle is not proof that you own it and without a steinberg account attached to the dongle it’s useless. Just buy Cubase 12. When you install it will pick up the plugins you have already installed.
Why have you installed Cubase 10.5 by the way since you don’t have a licence?