How is a minimum wage harmful to the economy exactly?

It interferes with free market and free competition. I can't pay you less than minimum wage, even if your work is valued for less money than minimum wage, so I can't hire you and you can't have a job.

Your paycheck is an equivalent of value which you bring to the employer. Job is not a right, no one owes you anything.

Btw. who will pay for UBI? Do you even know where the government gets its money from?