Before you proceed further, please place the frame on a FLAT SURFACE (this is crucial).
⬢All the components are cut or drilled by machine for highest precision, but with uneven tightening, it is possible to warp the frame.
Using your hand, try to wiggle the frame side to side and check, whether some of the front or back corners are lifting up or not.
Check the front and back corners only as the frame itself doesn't touch the surface.
⬢In case you find some imperfections, release the screws, press the extrusions against the FLAT SURFACE and tighten them again.
IMPORTANT INFO: the printer is capable of self-correcting a certain amount of the frame skew. Try getting the geometry as best as possible, however, if one of the corners keeps lifting up with values up to 2 mm (0.08 inch) you can proceed.