Page Layouts in SharePoint Online

Hello Teja1820 

you can't transform your classic custom page layout into a modern...

https://learn.microsoft.com/en-us/sharepoint/dev/solution-guidance/modern-experience-customizations-customize-sites#whats-not-supported-on-modern-team-sites

What's not supported on "modern" team sites

In numerous areas on the "modern" team sites, the typical customizations are not currently available. Further support will be available for some of these topics when they are ready to be released. Following is a list of currently unsupported customizations on "modern" team sites:

• Custom master pages; more extensive branding will be supported later using alternative options.
• Changing "modern" site to use "classic" seattle.master or oslo.master.
• Custom page layouts; we are looking to have support for multiple canvases in the future.

Here are a overview about modern: https://learn.microsoft.com/en-us/sharepoint/dev/transform/modernize-classic-sites

Here are more ressoruces about tranforming and the SharePoint Modernization scanner:

https://learn.microsoft.com/en-us/sharepoint/dev/transform/modernize-scanner

https://learn.microsoft.com/en-us/sharepoint/publishing-sites-classic-to-modern-experience

https://learn.microsoft.com/en-us/sharepoint/dev/transform/modernize-userinterface-site-pages-powershell

When you like to use the new power of SharePoint modern, then rebuild your page in modern, not with a transformation.

Best, Dave