The schematic calls for 6kΩ at 1% tolerance.
That allows a range of actual values from 6kΩ x 0.99 = 5.94kΩ to 6kΩ x 1.01 = 6.06kΩ.
Looking at some distributors' parametric search tools, 5.97kΩ is the nearest adjacent value known to be manufactured.
Of course, a 5.97kΩ 1% resistor would not satisfy the requirement, because at minimum this could be 5.97kΩ x 0.99 = 5.9103kΩ which falls outside the specified range of 6kΩ 1%.
But what about at tighter tolerance? A 5.97kΩ 0.5% resistor would have a range from 5.97kΩ x 0.995 = 5.94015kΩ to 5.97kΩ x 1.005 = 5.99985kΩ. Both these extremes are within the specified range. So a 5.97kΩ resistor is an acceptable substitute for 6kΩ 1%, provided its tolerance is 0.5% or better.
And at the time of writing, 5.97kΩ 0.1% is in stock at Digi-Key in 0402 size, which is more than sufficiently accurate.
This is quite a common trick when filling unusual values in a circuit - sometimes the nominal value you want is not available, but a nearby value is available with a tolerance tight enough to meet the requirements.
A better plan still, however, would be to find the datasheet for the PHY chip and check why this value has been specified. It is likely to be a reference resistor for a bias current or similar, and the chip datasheet will probably give the actual range that is acceptable.