Some of the warnings are caused by the fact that MOBI/KF8 files do not support all ePub tags, I.e. you'll have to live with them.
The errors are most likely caused by syntax errors in the source ePub that you didn't notice because Calibre will convert source files with serious errors in them without letting you know.
You may want to check the ePub for errors by clicking the big green checkmark button in Sigil to run Flightcrew and
check the file with ePubCheck Online
You also my want to check the stylesheet for errors by selecting the Validate with W3C option in Sigil.