attempt to index a nil value — the QBCore core object was nil

What you see

• A resource does nothing at all, or only some of its features work
• The F8 console shows the error once at startup, or every time an action is used
• It may affect every player or only players who joined at a particular time

What usually causes it

• The resource ran before QBCore was available and never retried
• The GetCoreObject retry was wrapped in CreateThread, which defers it — the rest of the file then registers with QBCore still nil, and it fails later at call time with no boot error
• A function above a `local x` declaration binds the nil global instead: it compiles clean and fails only when called
• Load order: the resource starts before qb-core in the config
• PlayerData accessed before the player has loaded in

Worth checking first

• Identify the resource and file:line from the error
• Check whether the error appears at resource start or only on a player action
• Check the resource's load order relative to qb-core
• Check whether it affects one player or everyone

Safe steps, one at a time

• Player-side: nothing. This is a server code problem — report it with the exact F8 text
• Staff: capture the full stack trace, the resource name and version, and whether it is one player or everyone
• Staff: check the core-object bootstrap pattern in the named file before changing anything else

When to stop and ask staff

Always staff. If it affects everyone, it is an incident: the resource is effectively down.