use PlayerEntity#getCapability and use CapabilityItemHandler#ITEM_HANDLER_CAPABILITY as parameter
example: https://github.com/Luis-st/Forge-1.16.5-36.0.1-mdk/blob/main/src/main/java/net/luis/cave/api/entity/player/PlayerManager.java
Kill your .gradle folder in your home directory and try having ti download again, seems your internet derped and corrupted something.
Else you can install gradle manually.