https://helpx.adobe.com/camera-raw/kb/camera-raw-plug-supported-cameras.html
The version of Lightroom Classic that you need is 8.1 or greater to directly support the D3500.
You seem to be at the equivilent of 6.6.1, if I remember correctly.
The current version of LrC is 11.1 or so, and it uses the subscripiton service.
Depending on which version of the Mac OS you are using, this can place limits on how far you can upgrade LrC.
If you don't want to, or cannot, upgrade to at least LrC 8.1, you can use the Adobe DNG converter to convert the NEF files to DNG files, then import the DNG files into LrC. From your description, you might already be doing this.
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