When the nozzle is too close or too far from the bed during the first layer. It can be caused by the following things.

• Z-homing sensor not properly setup, consult the manual of your printer, to properly calibrate the z-homing height.
• Z-home sensor not properly working, please read this article for more information.
• Print bed is not leveled, consult the manual of your printer for leveling instructions
• Printbed is not flat, it needs to be replaced. The required flatness tolerance for the whole print surface should be within +/- 0.1mm for good bed adhesion. For the TEC 4 series a flatness of +/-0.5mm is required, due to the surface measurment performed during calibration.