Gaps or ridges in the first layer

Try these in order

Most likely and quickest to check first. Change one thing at a time so you know what fixed it.

1. Bed not level, or not flat

   Gaps where the nozzle is too high and ridges where it's too low, in different parts of the bed, mean the bed isn't level.

   • Re-level the bed, heated to printing temperature.
   • Run or update the auto bed levelling mesh, and make sure the start G-code loads it.
   • Print a single-layer test square in each corner to see where the problem is.

2. Z offset wrong all over

   • Gaps everywhere: lower the nozzle. Ridges and a rough, shiny surface everywhere: raise it. Change it by 0.02 mm at a time.

3. Too much or too little plastic

   • Check the flow rate and the filament diameter set in the slicer (1.75 mm for most printers).