drawtext ffmpeg 'Unable to parse graph description substring'
For simple filterchains, where one filter operates upon the output of an earlier filter, no input pads (like [0:v]) should be used. Also, there is an errant colon at the end of the pad filter. It should be a comma instead, so
-vf 'pad=ceil(iw/2)*2:ceil(ih/2)*2,drawtext=text=&quot;hello&quot;:fontsize=90:x=20:y=20:fontcolor=white@0.2'

In drawtext, you may need to specify a font if fontconfig is not available on your system.