You are not alone. Until MIT discovers what is causing many to have this issue, follow this work around advice MIT App Inventor Issue (using Companion) - #17 by terry23462016.
This appears to be an issue involving latency in your ISP. Check the Use Legacy check box on your Companion screen before you use the QR Code and Chrome will work without getting stuck at 20%. If you do not check Use Legacy before pressing the QR Code button, you will get stuck.
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