Rename page URL from default to page name

I am new to wordpress and trying to rename my page URL from https://www.myURL/?page_id=21 to https://www.myURL/Page name.

I’ve tried under Settings > Permalinks, enter /%postname%/ in the custom structure. The result is https://www.myURL/pagename does appear but the page is blank with Error occurred: 404 – not found.

Is there something else that needs re-setting? I’m using Mantle theme.