Get a tube! Get a tube! It isn't too hard to install an inner tube, which
I've found easily available at hardware stores, tire shops, etc. Cheap,
works great, lasts long time, stop the aggravation! If you are not familiar
with tubes, maybe a quick stop at a tire shop (any auto tire shop has
experts in this department)

The softer (or harder ride if you want) rolls easier over small obstructions
or over tender new lawns better than any solid tire. Trust me. Been there,
done that, many times for friends and relatives.

Lynn the wheelborrow king