Please always show your sample data in text-tabular format in addition to (or instead of) the screen captures. A screen cap doesn't allow people to readily copy the data and run a quick test and thus decreases the likelihood of your question being answered. Just use 'Copy table' in Power BI and paste it here. Or, ideally, share the pbix (beware of confidential data).
You cannot reference a column in its own code. It's a circular dependency. Looking at how your column is built, I believe this would work:
Monthly Position =
SUM ( Table1[Billing] ) - SUM ( Table1[Collection] ),
Table1[Date] <= EARLIER ( Table1[Date] )
Please mark the question solved when done and consider giving kudos if posts are helpful.