My custom Taxonomy is not showing on my custom post types


I am using the Pinegrow Theme Converter. I did register a custom Post Type ‘Projects’ and want now to add a taxonomy for this to basically group these custom post types. I followed the documentation and it looks like the registration of the custom taxonomy was successful because when I click on my CPT I see the taxonomy in the dropdown. I was also able to add new tags here, that all worked.

However, if I go into my existing custom posts or create a new one, the taxonomy is not showing up here for me to choose one of the created tags to group the custom posts respectively.

Did anyone have that issue before and can tell me how I can fix this?