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It's because SageManifolds is not fully installed on your computer. Probably, you are using the limited version of SageManifolds which is included in SageMath 7.4. This version does not contain the metrics (nor any tensor field calculus). The next release of SageMath (i.e. 7.5) will contain the totality of SageManifolds. Meanwhile, you have to install SageManifolds atop SageMath 7.4 by following these instructions.

Another option is to use the SageMathCloud, where SageManifolds is installed system-wide in the SageMath 7.4 kernels of Jupyter notebooks.