Hi I am new here. Just wanted to know if I am welcome to be here.

You're definitely welcome here!

No, it isn't. Something I commonly tell people is that if your reader needs a dictionary to understand your work, you've failed at engaging them properly. If vocabulary is the problem, I would recommend searching up synonyms of commonly used words like beautiful, good, sweet, bad etc and finding better words for them. Every synonym has it's own nuances and can only be used in certain situations, so using those words correctly shows you have a good grasp of the language and also won't overload your reader. If you struggle with grammar rules or spelling, there are plenty of online resources for that too.

Also, welcome to TSR!

I wanted to take my writing to another level so I asked one of my friends what can he recommend. Guess what he said, I should master half of the words in the oxford dictionary, is that necessary if you have to write a book?