Sometimes when you load data in Power BI, the columns are not in the required format. For Instance if the data that is being loaded to Power BI was originally from a CSV file then all the columns would be of type text irrespective of whether a column was supposed to be of type Number or not.
When you have numbers that are saved as text it is Impossible to perform a mathematical calculation or use a DAX Formula which is supposed to take in numbers as Input. In this case you would have to click on Edit Queries and select the column that has a mismatched type. Activate the Model tab and click on Auto Detect. Power BI would automatically detect the original type of the column. And then Close and Apply.