Using a circular hollow section or any kind of tubes as member cross-sections lead sometimes to a singularity of the analysis model, especially when welding the tube along its length.
A typical example is the Connecting plate operation, while the type of connection of the tube to the plate is set to Notched plate.
This singularity is caused by the non-fitting division of the circular surface to discrete angular surfaces. To resolve this error, open the Code setup and try to change the default value 64 of the Division of surface of the biggest circular hollow member.
This helps to fit the finite element mesh of welds and the surface of the tubular cross-section and allows the analysis to be performed.