767 bytes in MySQL version 5.6 (and prior versions), is the stated prefix limitation for InnoDB tables. It’s 1,000 bytes long for MyISAM tables. This limit has been increased to 3072 bytes In MySQL version 5.7 (and upwards).
You also have to be aware that if you set an index on a big char or varchar field which is utf8mb4 encoded, you have to divide the max index prefix length of 767 bytes (or 3072 bytes) by 4 resulting in 191. This is because the maximum length of a utf8mb4 character is four bytes. For a utf8 character it would be three bytes resulting in max index prefix length of 255 (or minus null-terminator, 254 characters).
One option you have is to just place lower limit on your VARCHAR fields.
Another option (according to the response to this issue) is to get the subset of the column rather than the entire amount, i.e.:
ALTER TABLE `mytable` ADD UNIQUE ( column1(15), column2(200) );
Tweak as you need to get the key to apply, but I wonder if it would be worth it to review your data model regarding this entity to see if there’s improvements possible, which would allow you to implement the intended business rules without hitting the MySQL limitation.