Hi @Vinay Hadule ,
Currently, there is no such api to mark a file as final.
As a workaround, you could create a column on the document library called "_MarkAsFinal" (Yes/No). Then use API to update the column value.
Here is a similar question for your reference: https://sharepoint.stackexchange.com/questions/279073/display-marked-as-final-status-for-a-document-in-sharepoint-online-document-li
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