Creating seams and unwrapping tells Blender how to project the 3D surface of the clothing item in 2D. Before you can apply any texture appearance changes, you need to create a texture map, which is the specific 2D image you apply and save your textures to.
To create a new texture map:
With LongSleeve active, navigate to the Properties Editor > Material Properties tab.

Select the + New button.
Next to Base Color, select the yellow dot, then click Image Texture from the contextual menu.

Click the + New button.
- Name the texture image the same as your clothing mesh with a _TXT affix.
- Set the color to black or any base color of your preference.
- Keep the rest of the default settings.
- Press OK when completed.