Sometimes for an object the option to convert record detail to dynamic form is not available. Try to check these things are present on your page. If present, the option will not be available.
- Dynamic Forms is supported on record pages for custom objects, accounts (including person accounts), contacts, opportunities, cases, and leads only.
- Dynamic Forms doesn’t work in Internet Explorer 11. Users with IE 11 who try to view a page that uses Dynamic Forms get a page error.
- Dynamic Forms doesn’t work on pages based on pinned region templates and custom templates.
- Blank spaces aren’t supported.
There are some other limitations of dynamic forms which you can check here:
SFDCWallah 
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