I have rendered a video with kdenlive version 19.12 and it does not play on my Samsung TV (it does not recognize the codec).
I checked with "MediaInfo" the video rendered with Kdenlive and another video that does play and the difference is as follows:

Made with KDEnlive:
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L5.1
Format settings : CABAC / 4 Ref Frames
Format settings, CABAC : Yes
Format settings, Reference frames : 4 frames
Codec ID : V_MPEG4/ISO/AVC
Duration : 6 min 28 s
Nominal bit rate : 6 500 kb/s
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 25.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.125
Writing library : x264 core 158

Made with other aplication:
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4
Format settings : CABAC / 4 Ref Frames
Format settings, CABAC : Yes
Format settings, Reference frames : 4 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 3 min 47 s
Bit rate : 5 543 kb/s
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 25.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.107
Stream size : 150 MiB (96%)
Writing library : x264 core 155

How can I force in KDEnlive that the Format Profile be HIGH@L4?

Thanks for everything

Does your source footage is High@L5.1? Create a custom profile from MP4 and add following parameter: -profile:v high -level 4.0.

Thanks.