I tried to install my vegas movie studio on my computer. Even though I registered it in 2017 it says my activation code is expired since 2015. Please help me.
That would probably be Sony Vegas Movie Studio 9 which hails back to 2008 or thereabouts. In 2017, VMS9 was long gone as a current product so presumably it had been previously activated on an earlier computer. Sony Vegas activations had a limited number of activations before it hit a 'no more' barrier. To resolve this, you'd need to contact Vegas Support and request for the activation limit to be reset back to zero - https://www.vegascreativesoftware.info/us/support/interview/
And is it listed on your MAGIX Account > My products?
Note that "older versions of Vegas Pro [and Movie Studio] than 14.0 are licensed under a system (as previously used by Sony) which permits use on multiple computers by one user; after an unspecified number of installations on different or updated PCs you may need to contact support [advice here] to reset your "installation count". (As @Dexcon was minutes quicker to point out...)
Yes, I still see it under "My Products" so the download of the program is still available to me
That's good, but's its still more than likely that the activation limit has been been reached. If that is the case, you'll need to contact Vegas Support to reset the limit back to zero.
The MAGIX product is totally different. If you consider it, be sure to test with a trial version first. (Note the current cancellation terms and conditions: "Only for consumers who are habitually resident in a state that is a member of the EU or the EEA, this § 3 applies.")