Using the If function to set parameters on Text and Combo boxes on a userform

I am trying to use the if function on my userform to only allow certain values to be returned from the input of other values on the userform. I also want a message box to appear if certain fields are left blank based upon which data has been entered. Here is my code, but even when I have data input into the "Valve Position" combo box it still tells me "Data needed" Why?

Here is just an excerpt of my code:

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Any advice?

Thanks in advance.

I'm not sure what you mean by long expression?

I figured it out, I just did them separate like you said and it worked. Thanks

I figured it out. Thanks for your reply.