If you are getting this message "You must select a value in the [some] field in combination with the following dimensions values that are valid: [other] fields" it means somebody activated an account structure that requires you to use a defined set of values for the financial dimensions.
So, let me break down for you what this means: if you look in General Ledger -> Setup -> Chart of accounts -> Configure account structure you will see now some rules defined, one of them will be for the main account you are using in your operation (if you do not know what main account you are using in your operation, look for a following post on this).
This rule (or structure) defines some other financial dimensions, and possible values you should be using. You can better see the rule (and its options) by doing Edit and press the funnel icon on each dimension. Also, an important note: you have the option to Allow blanks for your dimension if you do not necessarily have to have a value for a specific dimension.