docker client is newer than server (client API version: 1.24, server API version: 1.21)

docker api

Docker is running on client / server model, each Docker Engine release has a specific API version.
The combination of the release version and API version of Docker is as follows:
https://docs.docker.com/engine/api/v1.26/#section/Versioning
According to the table above, the Docker API v1.24 is used in Docker Engine 1.12.x and the Docker API v1.21 is used in the Docker Engine 1.9.x. The server needs API version equal to or later than the client.
You have the following three options:

Upgrade the server side to Docker Engine 1.12.x or later.
Downgrade the client side to Engine 1.9.x or lower.
Downgrade the API version used at run time by exporting the DOCKER_API_VERSION=1.21
to environment variable on the client side.