What to do/check if I see such an error ?

Error: please, check the set up, lines could not be created due to the bad data or were already created in [...]

If you see the above error when trying to create billable lines, in the general case you should go to Jira Connect Setup and/or Project Card and check if all the data is set up correctly (depending on a particular Contract Type), e.g. for Fixed Price you may have to check the Project Invoice Date, for Monthly Pay - Ending Date, settings on the Project Resource Prices page, etc.

For more details check each Contract Type individually or create a ticket for support here.