Try NOT urlencoding the scope value. I was having this issue until i saw a previous post not encoding the value, that worked for me.
Notice no urlencoding, i got my access_token after making this adjustment.,
I found by not url encoding the scope if you are only requesting one scope solves the problem.
In my example, my scope value is simply `https://api.ebay.com/oauth/api_scope`
At first I was url encoding it as the docs suggest, but this solved my problem.
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