Welcome to KH!
Stockinette stitch does tend to curl and to flip a border in garter stitch. Making a deeper border can help fight this and blocking may help a bit. If it’s a sweater, the seams will help mitigate the curl.
What are you making? Can you give us a link to the pattern or a pattern name? (Not the whole pattern because of potential copyright problems)
What type yarn, wool, acrylic, blend?