Yes: Sibelius 7.x is incompatible with any version of macOS greater than 10.14 Mojave, owing to the changes Apple have made to the macOS since that time.
If you do not already have an Avid account at http://my.avid.com/ with you licence details for version 7.5, then you need to purchase the "Get current" upgrade deal from a reseller.
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Sibelius 2023.6/7.5.1/7.1.3/6.2/5.2.5, PhotoScore Ult 2020.1, Windows 10 64-bit 16GB. Desktop, and Microsoft Surface Book.
Please note that all Sibelius 7.x (and lower versions, including Avid License Control) is no longer supported on MacOS versions beginning Catalina up to the latest Sonoma version.