what are the challenges of the European Single Market

Title: what are the challenges of the European Single Market

Please work out the question of what are the challenges of the European Single Market. And their results/effects and so on. Please try to get into detail.

It is for a part of my essay, so I do not need any introduction about the European Single Market and so on.

I think it would be good to focus on Brexit and COVID-19. I need around 2.000 words

Reference style is up to you, but I need high quality sources and it must be easy to follow the sources/references afterwards for me. If you have questions please ask anytime.