I don’t remember my old iCloud account

Try these (all you should need to know is the password);

If you forgot your Apple ID - Apple Support

If you forgot your Apple ID password - Apple Support

If those don't help, then if you are the ORIGINAL owner, then see the 'Start a support request' section in:

How to remove Activation Lock - Apple Support

You can request an Online Activation Lock Removal HERE.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.