Comment by googie

For those seeking quick solution for missing codecs, here are bash scripts that use ffmpeg to convert any input clips (including these problematic h.265/h.264) to format acceptable for DaVinci

  #!/usr/bin/env bash
  
  set -euo pipefail
  
  INPUT_DIR="${1:-}"
  TARGET_FPS="${2:-30}"
  
  if [[ -z "$INPUT_DIR" ]]; then
      echo "Usage: $0 <directory with clips> [target fps (defaults to 30)]"
      exit 1
  fi
  
  if [[ ! -d "$INPUT_DIR" ]]; then
      echo "Error: directory does not exist: $INPUT_DIR"
      exit 1
  fi
  
  OUTPUT_DIR="$INPUT_DIR/conv"
  mkdir -p "$OUTPUT_DIR"
    
  EXTENSIONS=(
      mp4 avi wmv mpg mpeg mov
      mkv m4v flv webm ts mts m2ts 3gp
  )
  
  shopt -s nullglob nocaseglob
  
  for ext in "${EXTENSIONS[@]}"; do
      for file in "$INPUT_DIR"/*."$ext"; do
          filename="$(basename "$file")"
          name="${filename%.*}"
  
          output="$OUTPUT_DIR/${name}.mov"
  
          echo "Konwersja: $file -> $output"
  
          ffmpeg -y -i "$file" \
              -map 0:v:0 -map "0:a?" \
              -vf "fps=${TARGET_FPS}" \
              -vsync cfr \
              -c:v prores_ks -profile:v 1 \
              -pix_fmt yuv422p \
              -c:a pcm_s16le -ar 48000 \
              "$output"
      done
  done
  
  echo "Results in: $OUTPUT_DIR"

and then converting final exported video to h.265:

  #!/usr/bin/env bash
  set -euo pipefail
  
  INPUT="${1:-}"
  CRF="${2:-21}"
  PRESET="${3:-slow}"
  
  if [[ -z "$INPUT" ]]; then
      echo "Usage: $0 <input file> [crf=21] [preset=slow]"
      exit 1
  fi
  
  if [[ ! -f "$INPUT" ]]; then
      echo "Error: file does not exist: $INPUT"
      exit 1
  fi
  
  DIR="$(dirname "$INPUT")"
  FILE="$(basename "$INPUT")"
  NAME="${FILE%.*}"
  
  OUTPUT="$DIR/${NAME}_h265.mp4"
  
  ffmpeg -y -i "$INPUT" \
    -map 0:v:0 -map '0:a?' \
    -c:v libx265 \
    -preset "$PRESET" \
    -crf "$CRF" \
    -pix_fmt yuv420p \
    -tag:v hvc1 \
    -c:a aac \
    -b:a 192k \
    -movflags +faststart \
    "$OUTPUT"
  
  echo "Ready: $OUTPUT"