second hand Apple Watch won’t pair with iphone

I’ve bought a second hand Apple Watch, sometimes it’ll let me get through some sort of pairing stage.. then it will unpair. Sometimes it doesn’t even show up the 6 digit code. Can anyone help?

The lady I bought it off hasn’t had an iPhone for ages so doesn’t have access to it to disconnect anything? A lot of people have suggest go into settings on the Apple Watch but I can’t even get that far. I choose language - then country, then it gets to the pairing stage and I cannot get further.

TIA