You seem to have 3 different issues there, let's address them one at once:
All versions, (except the portable), will check for updates when you open CCleaner, unless you are not connected to the internet.
It won't update automatically though unless you have the "Apply these updates automatically" options selected.
As you can see in your screenshot there are two tick-buttons, one for Important Updates and one for the regular (monthly) Product Updates.
As far as I remember this is the first one to be deemed an Important Update.
For the Product Updates you can untick "Show me notifications for these updates" and you will not be shown that notification.
HOWEVER - v5.74 has currently been deemed an Important Update, and whilst it still has that Important status you will be shown the notification for it; That's the point of having different levels of Update.
If you don't install v5.74 you will continue to get that notification until the update is no longer flagged as <strong>Important</strong>.
(There is also a third, seperate, method for Emergency Updates, that is for critical security measures only so there is no option to delay them. That was introduced 3 years ago and has not had to be used yet).
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The Application error that some users are seeing is under investigation. (But see below).
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The licence not being accepted issue may be one of a couple of things.
What/which error message is being shown?
Check the information here for common problems, it's usually a typing error, a copy/paste including whitespace, or apermissions problem: <a href="https://support.piriform.com/hc/en-us/articles/204043844-How-to-register-and-activate-CCleaner-Professional" rel="external"> https://support.piriform.com/hc/en-us/articles/204043844-How-to-register-and-activate-CCleaner-Professional</a>
There is a slight possibility that the licence key issue and the Application error <u>may</u> be connected. (We have had one other report of this from before v5.47).
Can you sucessfully register the licence in older versions? or are you only being shown this licence error with v5.74?
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Note - v5.74 has a quick bugfix update and is now at v5.74.8198; you may want to try that new version if it wasn't the one you already installed.