Database already exists

as I said, the erpnext_shopify app does not exist in v11 any longer. The code of that functionality has been moved into the core code.

Ok. So can I know how to install a specific versions of ERPNext?

you don’t need to install a specific version, but to upgrade your current version to v11 (which will be a series up upgrade processes (v8>v9, v9>v10, v10>v11).

there have been a couple of Topics here on the forum lately about similar scenarios.