HiAnHuaZh,
You don't need a voucher to take practice tests. Read more about taking practice steps here .
Steps here
Go to https://www.certmetrics.com/adobe/candidate/exam_scheduling.aspx & expand practice tests . Click on
You will be landed on to practice tests portal (https://adobe.gmetrix.net/Tests/ChooseTest.aspx)
Select the 'Language' & then the 'Adobe Icon' below, you will see list of practice tests available
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Choose the relevant test,
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Select 'Testing Mode'
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You will see a survey,
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Once you fill in the details, you are taken to practice exam.
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Hope this helps.