what is wrong?
how do I solve it
In file included from sketch_mar30a.ino:6:
C:\Users\Bertil F\Documents\Arduino\libraries\LiquidCrystal_I2C/LiquidCrystal_I2C.h:81: error: conflicting return type specified for 'virtual void LiquidCrystal_I2C::write(uint8_t)'
C:\Program Files (x86)\Arduino\hardware\arduino\cores\arduino/Print.h:48: error: overriding 'virtual size_t Print::write(uint8_t)'
Well, you can do whatever you want, but you've been told several times that you did not supply enough information for anyone to even guess at what your problem might be. Nobody here is being paid to help you; we help because we like to help. In order to help, we need to know a few things about your setup, and if you are not willing to supply that information, you will not likely find anyone who is willing to help.
If you want to throw your Arduino in the bin, go right ahead. Or you could supply enough information to allow us to help. Your choice. It's just that simple.
What you'll find, when you get done reading, is that you are using an obsolete library. The write() method of the Print class is pure virtual. That means that any class, like LiquidCrystal_I2C, that derives from the Print class MUST implement write() AND must implement it with the proper signature. The write() method is no longer void. It is now size_t, and must return a value. The obsolete library you are using has the wrong signature for the write() method. It's a two line code change to fix it.