GST Element linking failed

[pvsnp9]

I have been trying to mix audio and vide using compositor and audio mixer. Here is the situation.
I have pointer for gst elements [appsrc, rtph264depay, avdec_h264, videoconvert]. I have added them to the pipeline. When i try to link them (in order) src.Link(dest) throws me the error gst_element_link_pads_full: assertion 'GST_IS_ELEMENT (dest)' failed. I checked the source type its gst.Element. The errors are follow:
(__debug_bin2560998154:2129271): GStreamer-CRITICAL **: 12:23:28.168: gst_element_set_state: assertion 'GST_IS_ELEMENT (element)' failed

(__debug_bin2560998154:2129271): GStreamer-CRITICAL **: 12:23:46.738: gst_element_link_pads_full: assertion 'GST_IS_ELEMENT (dest)' failed

2023/09/12 12:23:46 Linking: [rtph264depay] => [avdec_h264]

(__debug_bin2560998154:2129271): GStreamer-CRITICAL **: 12:23:46.738: gst_element_link_pads_full: assertion 'GST_IS_ELEMENT (src)' failed

(__debug_bin2560998154:2129271): GStreamer-CRITICAL **: 12:23:46.738: gst_element_link_pads_full: assertion 'GST_IS_ELEMENT (src)' failed

(__debug_bin2560998154:2129271): GStreamer-CRITICAL **: 12:23:46.738: gst_element_request_pad: assertion 'templ != NULL' failed'

(__debug_bin2560998154:2129271): GStreamer-CRITICAL **: 12:23:46.739: gst_pad_link_full: assertion 'GST_IS_PAD (sinkpad)' failed

Anything would help.