If you are building a custom theme and missing the align button on blocks (found on some blocks like headings, and cover) in your Block Theme (using html templates), or the align button does not have None, Wide, Full Width options, try the following:
In your page templates, move your “Post Content” block outside of any Group Blocks. It should not have any parent blocks. (I had mine inside a Group block, which disabled the align button, even though I set the Group block to use “Inner blocks use content width”. This particular set up disables the align button on blocks for some reason.)
In the Post Content block sidebar settings, toggle on “Inner blocks use content width”.
This should get the alignment button to show with the None, Wide, Full Width options on all blocks that have alignment options.