3060416 - SQL1390C The environment variable DB2INSTANCE is not defined or is invalid.

Following Db2 upgrade sql error SQL1390C is reporting on db startup.
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c:\Windows\system32> echo %DB2INSTANCE%
db2tst
    
c:\Windows\system32> db2start
SQL1390C  The environment variable DB2INSTANCE is not defined or is invalid.
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The db2diag.log reports the following:
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2021-05-25-15.18.45.009000+480 E3102F461            LEVEL: Severe
PID     : 1448                 TID : 1828           PROC : db2start.exe
INSTANCE: DB2TST               NODE : 000
HOSTNAME: VM-Temp1
EDUID   : 1828
FUNCTION: DB2 UDB, oper system services, sqloStartDB2Service, probe:20
DATA #1 : <preformatted>
Service start error.
Service name: DB2TST
Machine name: (null)
Error: DB2TST : The specified service does not exist as an installed service.
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DB2 LUW (DB6) any version

-1390 , KBA , BC-DB-DB6 , DB2 Universal Database for Unix / NT , Problem