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You don't give much details, so I will have to do some guessworks and my answer may not match your situation. If so, please provide more details.
Originally, when SSMS 21 was released, support for SSIS and maintenance plans was not present. This was added in later releases. However, it is an optional component, so just upgrading SSMS 21 is not enough.
You need to go to https://docs.microsoft.com/en-us/sql/ssms/download-sql-server-management-studio-ssms to download vs_SSMS.exe (unless you know where you have it locally). Start it, and then select Modify:
In the screen you get, select SSIS:
This will also select SSAS.