You are currently set as an exclusive member of your agency, and that's why you can't change the rate on your Freelancer profile. Please choose your Agency profile and go to Settings > Members and Permissions > click Edit under the Agency Freelancer column and change your status to a non-exclusive member. You should be able to change your rate from your Freelancer profile afterwards.
If you'd like to keep your exclusive member status, you can update your profile rate from your Agency profile by going to Find Work > Agency Roster > check the box next to your name and select Profile Rate from the Change Roster Settings drop-down menu.
Please discuss with the Agency admin or Agency owner regarding your Permissions under the Agency. You can find more information about it here
While on the Agency part of your account, please click on the Find Work - Agency Roster. There you should be able to edit your Profile rate.
Let us know if you need any further assistance.
I could find the agency that I have joined to in the find work tap, Could you please clarify how can I change the rate on my personal account to allow me to sent a proposals for any work,
I checked your account, and can see that you are an exclusive member of an agency. Since you are an exlusive member of the agency you are affiliated with, the agency owner, or business manager can update your hourly rate from their end. You may need to contact them directly regarding your concern.
I moved your comment to this thread so you can check the information I shared with the opening posted, and follow the instructions in order to update your profile rate.