The main issue is that many managers are selfish, if you are a technical talent they prefer to keep you locked in the technical side because you make their life

The main issue is that many managers are selfish, if you are a technical talent they prefer to keep you locked in the technical side because you make their life easier, you could state clearly your intentions for example: within the next three years I want to move from the technical side to the management side and define with your manager and future manager what should be done to achieve this goal making it clear that if it doesn’t happen you will be willing to seek other opportunities outside your company. It is your company that should provide guidance to what you should do to achieve this goal, some companies might offer you a master or MBA. Doing certificates and studying can help increase your knowledge and could be helpful if you seek such opportunity outside of your company but if you plan to stay in your current company it’s better to let them define what is needed so they don’t have excuses.