I love to make games. But I am not an artist. I never learned to make 2d or 3d models. I spent most of my time on programming. I know there are Game engines for non-programmers. But I am looking a way to create a decent strategy game (medieval theme) without spending time to learn how to model 2d or 3d models it is more time consume than learning how to program for an artist.
Your options are:
Source assets that fit your needs, either from free asset collections like OpenGameArt, or by purchasing the assets you want.
The downside with this method is that the selection may be limited, or assets you find might be mismatched with one another or not precisely the right look or internal setup for your needs.
Pay an artist to create custom art for your game, or to make corrections and adjustments to assets you've obtained through method 1 (providing the licence you have for those assets permits modifications).
Do not expect skilled artists to do this work for free or less than their time & skill is worth. That includes promising payment when the game sells (most game projects never turn a profit).
Choose a style that you can accomplish with your own skills and the source assets that you can find. That might mean this first game doesn't fit your ideal artistic vision, but with a little luck, you'll build up skills and resources to get closer to your original vision on your next project, or the one after that.
Your first game will not be your best game, so let it be what it is, and find a way to make that work stylistically within your constraints.
I note you've tagged this graphics-programming - this can include things like procedurally generating images and models through code, or amping up the look of plain assets through the use of shader programs. While a medieval setting is not exactly the easiest fit for these techniques, this is another area where you can shape your style to what tools you have available, finding a look of procedural graphics that you can run with to make a consistent visual identity for your game, using your own skills.