Trying to set up an account but dont have an account number

You should never use a previous tenants account, they should close it when they move out and you open a new one when you move in.

That way you avoid any potential problems with possible debit on the previous tenants account.

As you say that there is debt on the account then I’m assuming that this is a pay monthly account and not PAYG.

See this guide:

If it is PAYG then that’s a different guide.

whenever i try to set up an account it asks for an account number

Exactly how are you trying to set up a new account?
 

If you are meaning this then it’s the wrong place:

That is for accessing the account portal once you already have an account set up. (I agree that it could possibly be worded better there).

Read the guide above first then to set up a new account from scratch you would normally start on the “Get a quote” page:

https://products.ovoenergy.com/energy/customer-property

Enter you postcode and select the address from the list that comes up.
Answer the questions, select a tariff, enter your details, etc, follow the process through to set up a new account.

Once you have your new account set up then the debt on the old account is no longer your problem.
It may get a bit complicated later on, because you didn’t set up a new account when you first moved in and have been using the previous tenants account, so the previous tenant could try to say that you have built up the debt since moving in, But TBH that’s their own fault for not closing their account when moving out.
(I do realise that them not telling OVO that they were moving and closing their account might have been a deliberate move if there was already debt on the account at that time)