Every statement in Snowflake runs as a role, and roles decide what you can see and change. You have several. ANALYST reads SALES, WEB and RAW and owns a SANDBOX schema for your own tables. ENGINEER can change the data. HR_ADMIN is the only role that reads the HR schema. Switch with USE ROLE HR_ADMIN or the menu at the top right.
Without the right role, Snowflake says the object does not exist or not authorized. It won't tell you which, so that a role can't learn what it isn't allowed to see.
Queries that read or change data also need a warehouse: compute that starts when a query arrives and suspends when it's idle. You pay in credits while it runs. An X-Small warehouse uses 1 credit an hour, and every size up doubles that: Small 2, Medium 4, Large 8. Pick one with USE WAREHOUSE COMPUTE_WH.