Please review https://www.odoo.com/documentation/17.0/administration/hosting.html

You cannot upload an Odoo Online 17.1, 17.2 or 17.3 database to Odoo.sh - you can only upload an Odoo Online 17.0 database.  

If your database is an intermediary version you will need to wait until Odoo 18 is released.  You would then create a new Odoo.sh project using Odoo 18.0, upgrade your Odoo online database to Odoo 18.0 and change hosting at that time.

There is only a small window of a few months each year when you can change hosting from Odoo Online to Odoo.sh with an intermediary version.  This year, it will be after Odoo 18.0 is released and before Odoo 18.1 is released.

Odoo Online databases created with a Long Term Supported (LTS - which is 16.0, 17.0, 18.0 etc) version can be upgraded at any time during the year.

To create an Odoo Online database with an LTS version, use:

https://odoo.com/start?lts=1 

See also https://www.odoo.com/documentation/17.0/administration/supported_versions.html