You have two options: File now without the child and amend your tax return once the SSN is issued OR file for an extension and file your tax return when you do receive SSN. As a general rule, you have 3 years to amend a filed tax return.
(Please note that if you do file for an extension, it is for filing... not paying. If you have any tax due, it must be paid by April 15th, 2021 - unless you qualify for an exception, such as was granted to people affect by the recent Texas winter storm).
See Social Security Numbers for Children for how to obtain a SSN for your foster child. Needless to say, you will need cost approval and assistance to complete the application.
