CAUTION: SLIPPERY FLOORThe Arduino Library Manager installs the ArduinoJson version 6 by default.
However, using version 5 is highly recommended because version 6 is still in beta stage.
Open the Arduino Library Manager and make sure that ArduinoJson version 5 is installed.
This is a very common issue.
If ArduinoJson doesn’t appear in Library Manager, or if only old versions are listed, try to delete the local cache. This will force the Library Manager to download the library list from scratch.
For example, on Windows, you need to delete:
You don’t even need to close Arduino, just re-open the Library Manager.