How to get rid of the seam when creating a material or texture?

Hi guys so I just started learning Unreal two days ago. I encountered this problem when I tried to create my own texture material for building landscape. First off, whatever pictures I imported into Unreal, the engine always tells me " is not a power of two. Non power of two textures are never streamed and have no mipmaps. Proceed?" I dont quite understand what does this mean, is it the problem of my pictures? Second, when I create a material texture, this is always a seam on the texture, I attached some screenshots, if you look close, they are different from the default texture that Unreal provides to us. My own texture is somehow very small so when I put them all together they make up very annoy and obvious seams. Could anyone tell me how to fix these problems? Thank you guys, I am just a beginner of UE4, I really could use some help here.

My texture is the white, snow like one, the dirt texture is the default, I also attached my nodes.

Hello AltWing!

Textures need to be in power of two.
Which means their resolution needs to be 2x2, 4x4, 8x8, 16x16, 32x32, 64x64, 128x128, 256x256, 512x512, 1024x1024, 2048x2048, 4096x4096, etc.
(or an alteration of that like 2048x512, 1024x32, etc)

If there are seams in the texture it means you might want to make the textures (more) seamless.
Tutorial for it: Seamless Textures With Photoshop - YouTube

Suggested sizes for terrain textures are 1024x1024 (small), 2048x2048 (regular), 4096x4096 (great).

Thank you! I followed your solution and fixed it!