As @chandv wrote, open savnw network. The limits of the current version is shown with menu: "File - File information - List the number of system components".
For the 50 bus version:
PTI INTERACTIVE POWER SYSTEM SIMULATOR--PSS(R)E Xplore TUE, SEP 19 2023 19:51
PSS(R)E PROGRAM APPLICATION GUIDE EXAMPLE
BASE CASE INCLUDING SEQUENCE DATA
BUSES PLANTS MACHINES WIND MACHINES MACHINE OWNERS
TOTAL 23 6 6 0 14
MAXIMUM 50 15 20 5 40
S H U N T S INDUCTION
FIXED SWITCHED LOADS MACHINES
TOTAL 5 0 8 0
MAXIMUM 50 10 100 20
T R A N S F O R M E R S
BRANCHES TWO-WINDING THREE-WINDING ZERO IMPEDANCE BRANCH OWNERS MUTUALS
TOTAL 34 11 0 0 35 6
MAXIMUM 100 20 5 10 200 20
MULTI-SECTION LINE
GROUPINGS SECTIONS AREAS ZONES OWNERS TRANSFERS
TOTAL 2 4 3 4 7 4
MAXIMUM 5 15 10 25 25 15
2-TERM. DC N-TERM. DC VSC DC FACTS DEVICES GNE DEVICES NCS TABLES
TOTAL 0 0 0 0 0 0
MAXIMUM 5 2 5 5 2 8
SWITCHING DEVICES
SUBSTATIONS NODES TERMINALS IN SUBSTATION TOTAL
TOTAL 0 0 0 0 0
MAXIMUM 16 200 200 200 200
It's not only buses, but all other components such generators, loads, etc. are limited to different dimensions. If one of the limits is exceeded, such messages may come. You may try opening inbuilt cases such as savnw.sav to check.