Message on my computer
You are opening the library “Photos Library.photoslibrary” for the first time. Are you sure you want to open this library?
Is there any chance of losing my photos if I click OPEN?
iMac 21.5″, macOS 10.15
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The message is normal if you are in fact opening that Photos Library for the first time.
It not directly related to your question but if you do not routinely back up your Mac then the prospect of permanently losing all the information on it does not concern you. To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support.