A DCP-Digital Cinea Package is a folder with a collection of specialized files. With Davinci Resolve Studio, you can export a DCP for playback in theatres that support the DCI standard DCP files.
Your Resolve project (timeline) should be at one of the 2k DCP aspect ratio and resolutions 2k DCI Full (1.9), 2K DCI Flat (1.85), 2k DCI Scope (2.35), not full is simply called 2k DCI
Most theatres are still 2k, not 4k, so at this point its not super useful to create a 4k DCP
see here for more info on the DCP format on
https://en.wikipedia.org/wiki/Digital_Cinema_Package
If your film is not one of these ratio, it must be either letterboxed or pillarboxed inside of these ratios