Texture mapping isn't clear at all.

Blockbech does not handle *.jar files, you might want to say *.json for blocks/items. Create your desired block or item with all the textures you need. When you are happy with the results (make a backup now ... save the model as a *.bbmodel, just in case), select the entire model (make sure you select all the blocks used to create the item or block - CTRL+A) and click on create texture. Change the values (if needed) and click confirm, this will create a uv mapping (new texture) with all the textures used but combined into one. Now export the recently created texture only first, then the model. Finally import the model to MCreator and when prompted for the texture, you will only need to select one texture.

Happy to help.