How to correct this? Why does this happen?

When I export vídeos on Da Vinci, sometimes this happens and I don’t know why.
Can you see the strange lines in the image? Like something does not suport the quality, I don’t know.

Can someone help me?

Thank you so much!!!

called banding. google it.

probably 8bit footage, might look like that in the original or you've brought out the banding via colour grading.

Can someone help me?

Assuming the issue is not because of low quality footage, any 10 bit (or higher) export option should help.

When I export vídeos on Da Vinci, sometimes this happens and I don’t know why.
Can you see the strange lines in the image? Like something does not suport the quality, I don’t know.

Which hardware (CPU, GPU, RAM, disk I/O)? What specific version of Resolve? What OS? What source file codec? What timeline settings (res and fps)? What framerate? How does your drive rate with Blackmagic Speed Test? How is the drive formatted and connected? Which GPU drivers are installed? What format are you trying to render to?

A lot of people seem to be working with 8-bit, Long-GOP H.264 material these days, and it's not a good format for post in a lot of ways. 8 bits are not enough to capture a wide range of color and B&W information, and you tend to run into "banding" and other problems as a result. Better cameras will shoot in 10-bit or even 12-bit color depth, and you get a lot more dynamic range and more information to work with that way.

There is an OFX DeBand plug-in you might try and see if that helps, if you're seeing horizontal lines or bad gradations in your exports.

In the end it boils down to the old GIGO rule: https://en.wikipedia.org/wiki/Garbage_in,_garbage_out

H.264 in 8 bit offers very limited flexibility in grading. Shoot a camera profile you like, expose carefully and get the color temperature right. If you don't like what comes out of your camera, there's little hope to 'fix it in post'.

Finally, YT makes things even worse.