When developing with 4D, it is important to be aware of the main reasons why you may not be able to access Design mode:

• Your database has allowed Design access only to a certain group, and you (as the current user) are not part of that group. More information here: https://kb.4d.com/assetid=78364
• You are running the database as compiled structure. While running it as compiled, 4D no longer has access to the interpreted code of the structure, so it is not possible to use Design mode features.
• Your current version of 4D does not have an active license to allow Design mode access. Please remember to activate licenses for any newly-installed versions of 4D, before trying to access Design mode for your database.