You have a few different ways to determine the last day of the month.
Determine if today is the last day of the month, such as when you schedule a flow to run daily:
If(Month(Today()) = Month(Today()+1), "Not Last Day Of Month", "Last Day Of Month")
Subtract from first day of the month, to get the last day of the month, for when you want to run flow on a schedule:
subtractFromTime(startOfMonth(addToTime(utcNow(),1,'month')),1,'day')
The Time intelligence functions (DAX) has loads of ways to determine the time/day, including ENDOFMONTH, and ENDOFQUARTER.
If you could provide an expanded screenshot of your Flow and steps, your Flow run history, and of any detailed error messages you're receiving we could likely better assist you. Also, for the best results, you may want to review How to write a good forum post.
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