Q: this item is temporarily unavailable

Over 50 computers proudly were the nameplate "MacBook Pro".

¿Which one of those do you have?

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There are thousands of items available in the Mac App Store.

¿ which item is giving you this issue?

Just what are you doing when yo get that message?

What model Mac?

What OSX version was on the Mac?

 menu > About this Mac

(please do NOT include the serial number in your reply)

To identify MacBook Pro model

https://support.apple.com/en-us/HT201300

what Processor or Mac Model is this?

what screen size?

You appear to have a mid 2009 MacBook Pro. Those came with either Leopard or Snow Leopard.

You have not yet answered my other question "Just what are you doing when yo get that message?"

It could be that you booted to Recovery and tried to download/install Lion.

Lion was a paid update and is lock to the Apple ID that originally purchased it.

Are yo the original purchase of this mac? Did you enter the Apple ID originally used to purchase Lion?

Is that the only Apple ID you have?