Unfortunately it is not possible to set alt text for images in the Block Editor, unless you use a HTML Block.
As an alternative, you can create the template using the Block Editor then switch it to the Freestyle Editor to add alt text for images. Please keep in mind that once you do this, you will not be able to revert back to the Block Editor. We suggest first cloning the template then changing the clone to the Freestyle Editor, this way you will always have the original template.
You can also try building your template in our Drag & Drop editor.