database is not accepting commands

We have 1.5 TB database that's shown an error and block all commands.
The error is :
"ERROR: database is not accepting commands to avoid wraparound data loss in
database "dbname"
HINT: Stop the postmaster and use a standalone backend to vacuum that
database.
You might also need to commit or roll back old prepared transactions."

I tried to do vacuum in the backend mode. Also I tried to set
zero_damaged_pages = on then do the vacuum again but same error appeared.
The error appeared after two hours of vacuuming where verbose shows passing
tables.

The cause of error could be :
1- a lot of insertion queries for not existing table
or
2- truncating data with 40 GB in a week,
we used to do that but this is the first time we had this error.
also, autovacuume is turned on.

Please , I need help or any suggestion?

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