First of all, always try opening the end point url in new tab of browser, if it doesn't give any error then, it is an indication that we can use it. If browser displays something like "This site can't be reached", then it is not a valid end point or server is not up and running for respective end point.
In case of toolsqa website, in starting few pages endpoints are not working. I sent a message to Virender to look into this issue, but unfortunately no response.
I suggest you to learn rest assured from makeseleniumeasy website as per my personal experience.
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