First of all, as Doris said, you need meet the most basic conditions for installing the azure-devops extension, that is, the version of az cli should be higher than 2.0.69. But as far as I know, this should not be the cause of issue you encountered.
azure cli along with extensions are all built with python. So you will find that "Error detail: Expecting value: line 1 column 1 (char 0)" is a normal JSONParseDecode error if you google it. It has many causes, like empty response, non-json response content, 500/503 error and etc. Without internet trace, I'm afraid it's hard to find the exact cause of it.
Work around:
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To work around this issue so that you can use az devops command to achieve your goal, you can follow below steps:
1) Install the corresponding whl file into your local machine.
2) Retrieve the downloaded whl file location, then run this command from your command line(Here I would show you the sample script I used):
az extension add --source C:\Users\merlinl\Downloads\azure_devops-0.18.0-py2.py3-none-any.whl
3) Now you will see that the azure-devops installed successfully.
