Hello Abdullah,
In the form you opened you have to open the main account section first and then select "edit".
This will open the main accounts form.
Example:
In this form you need to select / switch the company for which you want to specify the financial dimensions.
Example:
You can either setup a "default" financial dimension or one that is "fixed".
Please not that fixing a financial dimension in the chart of accounts overwrites any other financial dimension setup that you make e.g. at the customer level.
If you are new to AX and have not worked with financial dimensions before I suggest that you first take a look at the financial dimensions hierarchy before trying to fix financial dimensions at the chart of accounts level.
All the best,
Ludwig