Hi @Dil123,
Thank you for posting in this community.
By design, we can't convert classic site to modern site in SharePoint on-prem by using PowerShell.
There is no such PowerShell available for SharePoint on-prem, only available for SharePoint online.
We are always looking for ways to improve SharePoint, and your feedback and suggestions are very valuable.
We suggest that you can make suggestions on this SharePoint Feedback portal for SharePoint On-prem PowerShell from the classic experience to the modern experience. Your contribution will be highly appreciated.
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