How can a government save, if it has deficit?

Government can have savings while having deficit because we are just talking about savings not net savings.

For example, imagine that government has zero tax revenue $$10$ spending and $$10$ of public investment. In this case the government is running deficit of $$20$ yet it is also saving through investment. Due to the deficit being larger than saving on net basis government still dis-saves but saving is not zero.

So it is completely possible to save and run a deficit at the same time.