ROTAX 912 low fuel pressure at take off

The Rotax 912ULS operator manual states the following:

Max fuel pressure. 0.4 bar (5.8 psi) & (0.5 bar (7.26 psi) - applicable only for a fuel pump from S/N 11.0036

Min fuel pressure 0.15 bar (2.2 psi)

https://www.rotax-owner.com/pdf/OM_912_Series_ED4_R0.pdf

At no point during engine operation should the fuel pump produce 2.2 PSI or lower.

Is the aircraft equipped with an AUX electric boost pump?

Check out  the earlier post/conversation "Pilot Jet#35"

Yes, the aircraft has an AUX boost pump. Another question - while in take-off is it mandatory to use the AUX boost pump as in addition to the mechanical one?

That will depend on the aircraft's operator's manual and checklist. In most cases, the AUX pump is turned on for take-off and landing. 

AUX boost pumps don‘t seem to be „mandatory“ in general terms - too many planes fly without having one installed. It‘s supposed to provide some extra safety (better or more stable fuel supply). Thus, to me it makes perfect sense to use it than to just have it on board, especially when taking off an fuel demand is high.

;-)