I found another post and someone posted this for fix and it worked for me. Not sure if it would help you, but I thought I'd still post.
"Had the same issue on the police4 slot, here is what fixed it for me.
Goto Rockstar Games\Grand Theft Auto V\mods\update\x64\dlcpacks\mpluxe\dlc.rpf\common\data, open the carvariations.meta and add the following part.
<Item>
<modelName>police4</modelName>
<colors>
<Item>
<indices content="char_array">
134
134
156
156
</indices>
<liveries>
<Item value="true" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
</liveries>
</Item>
<Item>
<indices content="char_array">
134
134
156
156
</indices>
<liveries>
<Item value="false" />
<Item value="true" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
</liveries>
</Item>
<Item>
<indices content="char_array">
134
134
156
156
</indices>
<liveries>
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="true" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
</liveries>
</Item>
<Item>
<indices content="char_array">
134
134
156
156
</indices>
<liveries>
<Item value="false" />
<Item value="false" />
<Item value="true" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
</liveries>
</Item>
<Item>
<indices content="char_array">
134
134
156
156
</indices>
<liveries>
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="true" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
<Item value="false" />
</liveries>
</Item>
</colors>
<kits>
<Item>157_massacaro2_modkit</Item>
</kits>
<windowsWithExposedEdges />
<plateProbabilities>
<Probabilities />
</plateProbabilities>
<lightSettings value="1" />
<sirenSettings value="13" />
</Item>