if the installed version is 10.0 and later you can use it for Odoo 15, give the port, username, and password in the next step (click next without selecting PostgreSql Database)

To Install the new version :

- Goto https://www.postgresql.org/download/windows/

- Install PostgreSql

- By default, the only user is 'postgres' but Odoo forbids connecting as 'postgres', so
you need to create a new PostgreSQL user:

- Create a Postgres user with a password using the pg admin GUI:

Open pgAdmin.

Double-click the server to create a connection.

Select Object ‣ Create ‣ Login/Group Role.

Enter the username in the Role Name field (e.g. odoo).

Open the Definition tab and enter the password (e.g. odoo), then click Save.

Open the Privileges tab and switch Can log in? to Yes and Create database? to Yes

- Add PostgreSQL’s bin directory (by default: C:\Program Files\PostgreSQL\<version>\bin) to your PATH.

After this run Odoo.exe

Give the port, username, and password of the Postgres installed in the next step (click next without selecting PostgreSql Database)

OR

To enable Postgres check box during installation you need to completely uninstall the previously installed Postgres if it's not required.