Im designing a vertical vessel, 2 cylindrical sections flanged in the middle of the total height.. then this advice appears...
[/color]** Warning: Unity Check for Rigging Exceeded 1.0 !
[color:#FF0000]Maximum Unity Check is 1364.4904 at elevation 22.5000 ft. - Must be <=1

But if I eliminate the flanges and instead of it I consider just the weights (inputs-->weight) then this WARNING dissapear.

Do you know why is this happening?

Rigging A/s is done to check vessel in bending when it is lifted from horizontal position to vertical position. for this you have to enter tailing lug & lifting lug elevation by going in loading tab in PV Elite in Miscellaneous options.
if you dont want to check your vessel in rigging then delete those elevations. if you also want to check in rigging, then you must have to increase thickness at the elevation shown in warning.

But this ratio is generally near around 4 or 5. in case it is very high.

/kindly attach your model for more information.