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I'd made my first attempt to boost post (just trying to figure out how Ecency works), but the "Next" button (located right below the text "Drag the slider to adjust the amount.") would gray out and couldn't be clicked.

Gray out means your balance is not enough for other amounts, please check your Points balance and how much Promote/Boost is required.

Thanks for your kind reply, Ecency. I forgot to input the username (I added only the post link without the username) and that happened to be the culprit behind the gray out.