Error: java.lang.UnsupportedOperationException: Can't convert to dimension: type=0x3

You need to define your dimensional resources as dimens not strings. So, move your @string/marginbutton from strings.xml to res/values/dimens.xml as

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <dimen
        name="marginbutton">8dp</dimen>
</resources>

Your error basically complains about passing a <string> resource where a <dimen> is expected.