Extension speaker question regarding wattage

I have the Roland Cube 60 and you can connect this to an external speaker. The speaker I have is a 2X12 cabinet with a max wattage of 150. The Cube says that the external speaker should be 60W max. Would this cause any problems to the amp and/or cabinet?

No.

That's right. With 60W output, the power is split between the 2 speakers. So each speaker would be 30W at full output. There won't be a problem if the speakers are able to handle more than that amount. You do need to keep the impedance rating matched or at least have the speakers at a higher impedance than the output of the amp. If the speakers are lower impedance it will put too much of a draw on the output transformer and may cause damage to the amp.