Well, as per the title. Watch your whitespace.
For example, I was getting this error on the checker and tried to figure out what is wrong with the reference name.
When you use expressions you have to type things in. This is where you have to extra careful.
It seams to look OK but in reality it’s a bit not OK, we just can’t see it straightaway.
There is an extra whitespace after the Get PNLM Delay. You can barely see it but it’s there.
And it may cost you some time and extra grey hair. Let’s remove it!
Happy days 🙂
Something Awesome About ... Power Platform and BizApps..Dev and Ops 
It is good to know that a pesky white space is responsible for your problem. I wish I was that lucky.
I have a situation that demonstrates that expressions are an utterly poor and miserable implementation by Microsoft. I have an HTTP connector followed by a Set variable to fetch the results into a variable and replace the double-quotes with single quotes in the output of the HTTP step using an expression.
I went back and renamed the HTTP step, and changed my reference in the subsequent expression below. I didn’t type the expression, but pointed to the “Body” reference of the HTTP step. But no matter what I did, the Flow error still showed me the older name of the HTTP action step.
Long short – it works if I refer to it without any modifications. But if I put it into an expression such as replace, the flow error shows up preventing further progress.
The effect is magnified inside of an App for each construct and it is hard to get rid of this nonsensical situation. I can’t believe such basic things don’t work in something named “Power Platform”.
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So sorry you have to go through this. But low code or no code development is still a development with all related “perks”. The good news you always can use multiple tools/ways to solve the same problem.
The most powerful tool in Power Platform is you.
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