this means that you have reached the Max Supported Mux VLANs :
Feature Limitations
Table 2 describes the specifications of the MUX VLAN.
Table 2 Specifications of the MUX VLAN
Item Specification
Maximum number of principal VLANs on a switch
128
Maximum number of separate VLANs in each principal VLAN
1
Maximum number of group VLANs in each principal VLAN
128
NOTE:
Each principal VLAN supports a total of 128 separate and group VLANs. That is, if one separate VLAN is configured, a maximum of 127 group VLANs can be configured.
The VLAN ID assigned to a principal VLAN cannot be used to configure VLAN mapping, VLAN stacking, super-VLAN, or sub-VLAN.
The VLAN ID assigned to a group or separate VLAN cannot be used to configure a VLANIF interface, VLAN mapping, VLAN stacking, super-VLAN, or sub-VLAN.
Disabling MAC address learning or limiting the number of learned MAC addresses on a port will compromise the performance of the MUX VLAN feature.
MUX VLAN and port security cannot be configured on the same port.
MUX VLAN and MAC address authentication cannot be configured on the same port.
MUX VLAN and 802.1x authentication cannot be configured on the same port.
When both DHCP snooping and MUX VLAN are configured, if DHCP snooping is configured in the subordinate VLAN and DHCP clients are configured in the principal VLAN, the DHCP clients may fail to obtain IP addresses. In this case, configure the DHCP server in the principal VLAN.
After the MUX VLAN feature is enabled on a port, VLAN mapping or VLAN stacking cannot be configured on the port.
You can create a VLANIF interface for a principal VLAN. However, only the following switches support the creation of a VLANIF interface for a subordinate group VLAN or separate VLAN: S1720GFR, S2750EI, S5700LI, S5700S-28P-LI-AC, S5700S-28P-PWR-LI-AC, and S5700S-52P-LI-AC.
When MUX VLAN is enabled on an interface and a PVID is configured using the port trunk pvid vlan command, do not configure the PVID as the ID of the principal VLAN or subordinate VLAN of the MUX VLAN. For example, VLAN 10 is the principal VLAN, VLAN 11 is a subordinate group VLAN, and VLAN 12 is a subordinate separate VLAN. After the port mux-vlan enable 10 command is used on the interface to enable MUX VLAN, do not run the port trunk pvid vlan command to set the PVID to VLAN 11 or VLAN 12.