Hi @detas979
Yes, you can do this. As part of the visa process for family reunification the authorities will then ask why you are only at that point applying for reunification and will wish to see evidence of how you maintained contact with your child while you were in Germany. They will also require proof that you have sole custody of the child.
You may be able to bring your child to Germany with you as a family member while doing the Ausbildung, if this is a consideration for you. You will be entitled to child benefit (Kindergeld) of 250€ as well as an allowance for single parents (Unterhaltsvorschuss) of 227€ per month. Depending on your situation, you may be entitled to additional benefits such as extra child benefit (Kinderzuschlag) and rent supplement (Wohngeld). Having proof of sufficient income is an important requirement if you wish to have your child also receive a visa.
Best,
Éanna