@henk Seeing as you CRT has Scart inputs it would be a downgrade to pair it with a Composite cable when the CRT has support for RGB. Look into PI2SCART or Recalbox RGB Dual instead and save yourself the hassle and trouble.
I don't think you'll be able to use Ubuntu 16 to install RetroPie, even if you check out the 4.5.1 version - most of the packages will not work with that old Ubuntu version.
You can try installing the current Debian release or a more recent (18.04) Ubuntu version and install RetroPie as explained in the docs.