3DPrinting

I'm in agreement with the others. This is a printer issue, not a model design issue. Any current printer in good working order and running non-insane settings should be able to print a 90 degree inside corner like that with no problem.

Some possibilities:

Your Z offset may be set too high, so that your first layer height is too tall. This will result in the first layer's extrusions not sticking to the bed and each other, peeling off in strings like you see here.

Flip this over and show us the bottom of it. The effects of a too-high first layer should be readily apparent. That's where my money is.

Your printer may also be attempting to round the corners too fast. You could slow down your print speed, or adjust your linear advance settings. If you are using Prusaslicer or a derivative thereof (Orca, Qidi, etc.) there are built-in calibration prints you can run that will provide you a range of values to inspect my physically printing them, and allowing you to choose from the value that produces the best looking result. Ideally your linear advance/pressure advance setting should be tuned for each spool of filament, but in reality most people (myself included) don't bother until they observe an issue. I use the same settings for all PLA, and a different set of settings for all PETG, and another for ABS, etc.