Code to grey out and lock/unlock certain cells

Hi everyone

I am relatively new to VBA and have attempted to write a code (see below) to grey out and lock certain cells depending on whether or not a certain value is entered to a dropdown list.

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This seems to work fine until I protect the sheet, after which as soon as I change the value in cell "D24" I get a run-time error 1004 - Application defined or object defined error, with the debug highlighting this line:

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Any ideas how I can fix this?

Thanks in advance for the help!

Something like this

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