If you receive the error: Contract Period not found when trying to post a batch, please check that the period has been created and has been opened. To check that:
Go to Contract Status
Select Setup
Select Setup Maintenance
Click on Global Valuation Dates
Tick Open next to the affected period or click the + icon to add the period if it's not already present
Once the period is confirmed to have been created and open, please try reposting the batch.