After isntalling the package on M2.3.0 CE, please edit code in file 

Bss/MultiStoreViewPricingCatalogRule/Model/Indexer/ProductPriceIndexModifier.php
from

public function modifyPrice(IndexTableStructure $priceTable, array $entityIds = [])

to

public function modifyPrice(IndexTableStructure $priceTable, array $entityIds = []) :void