Lidl complaint: Earned rewards that fail to reward

I made numerous purchases in Dec, which resulted in my earning 4 rewards which expire 1723. Today I was only able to make use of 2 of them. I spoke with an employee who checked her phone info to help me find acceptable items in the produce department when my initial selections were not in stock. I selected products but did not get the reward benefit when checking out. A manager was called when I opted to return those items that did not have the price I was expecting when trying to use the rewards. I was informed by her that I needed to purchase $50 worth of items at the current time point of purchase in order to get the benefit. I tried to explain that I already made the appropriate amount of purchases in order to get the reward offer. I was frustrated by this experience and was not encouraged to make Lidl a regular stop for shopping. Please advise how I can purchase products of my choosing when earning rewards, such as with equal substitutions or rain checks as options when product availability is an issue. Thank you for your help in resolving this unhappy rewards shopping experience at Lidl, which was anything but rewarding.