Creating a summarizecolumns filling blank data with zero - powerbi

Question

I'm kind of new with DAX and I'm currently stuck with a problem.
I have two tables Fiscal Calendar and Customer. I created a relationship using the date columns from both table.
The customer table contains records and I would like to show on a matrix if a record exist or not on specific dates (or fiscal weeks) like in the image below.
I made a summarizecolumns table (see the dax sentence below) however, what I need is if the record doesn't exist on specific date/week, I want to fill it with zero value instead of having it just blank. I tried many things already but until now I couldn't find any solution. If anyone is willing to help me I would really appreciate it.
Adherence =
var customerNames = VALUES(Customer[CUST_NAME_EXT])
var customerNumber = VALUES(Customer[CUSTOMER_EXT])
var selectedDates = VALUES('Fiscal Calendar'[fiscalDate])
var filterCustomerNames = FILTER(Customer,Customer[CUST_NAME_EXT] in (customerNames))
var filteDates = FILTER('Fiscal Calendar', 'Fiscal Calendar'[fiscalDate] in selectedDates)
var filterCustNumber = FILTER(Customer, Customer[CUSTOMER_EXT] in customerNumber)
var t = SUMMARIZECOLUMNS(Customer[CUST_NAME_EXT],Customer[CUSTOMER_EXT],'Fiscal Calendar'[fiscalYear],'Fiscal Calendar'[fiscalWeek], filterCustNumber ,"isAvailable", 1)
return t

Answer 1

First, I want to give you kudos for using SUMMARIZECOLUMNS. If I understand you correctly, you were trying to write DAX as if it is an SQL query, but it is not the case. To know the difference between them, you can read my answer to {How to write SQL in DAX?} question using this link:
How to write SQL in DAX?
Still, you can do it more easily by using the Power BI modeling strength because you said you had created a relationship using the date columns from both tables, Fiscal Calendar and Customer.
Second, to achieve your requirement, you need to create a flag to show 1 if a customer exists on a date or week and 0 if not, which, thankfully, can be implemented for any segregation level (columns) in your Fiscal Calendar table, just by two simple measures, which are as follows:
Count Rows = COUNTROWS(Customer)
Adherence = IF(ISBLANK([Count Rows]),0,1)
Finally, as your image shows, you will use the simple Adherence measure to give you the needed result.
In the end, I want to give you a helpful tip; I see you are trying to create a custom heat map using the Power BI matrix, which is very cool. Please use this link as a reference for more design ideas for what you are trying to achieve. The link is:
https://sqlskull.com/2020/12/17/implementing-a-table-heat-map-in-power-bi/#:~:text=A%20Heat%20map%20is%20used%20to%20visualize%20the,to%20apply%20some%20conditional%20formatting%20in%20matrix%20table.
I hope I helped in a way; if so, please mark this as an answer and vote for it :)