If you UTFSE in this forum for "nabh3cn
or cyanoborohydride", you will find almost exclusively posted procedures which involve reductive amination of various tryptamines with formaldehyde and NaBH
3CN - all of them have yields of 60% or more.
KrZ's synthesis in particular is not optimized, he runs the reaction for a far longer time than necessary, and in the first procedure he posted he used far too little formaldehyde for the reaction to go to completion. Try the procedure below instead, if you don't have any silica gel for chromatography, then you can try repeated recrystallization instead to purify the product, either from hexane (petroleum ether), ethyl acetate (EtOAc) or am mixture of both.
DMT by reductive methylation of Tryptamine with 37% Formaldehyde and NaBH3CN in MethanolSynthesis according to a general procedure in J. Med. Chem. 37(19), 3029 (1994)
(https://www.thevespiary.org/rhodium/Rhodium/pdf/reductive.tryptamination-2.pdf)Tryptamine (1.12g, 7 mmol), Sodium Cyanoborohydride (0.88g, 14 mmol) and Glacial Acetic Acid (2ml, 35 mmol) was dissolved in 110ml Methanol at 0°C, and a solution of 37% Formaldehyde (1.4 mL, 18.5 mmol) in 15ml Methanol was added dropwise over 20 min, and the resulting solution was allowed to stir for 20 min at 0°C and 2.5h at room temp. The methanol was evaporated under reduced pressure, and 80ml 25% aqueous potassium carbonate was added and the solution extracted with 2x125ml EtOAc, the extracts washed with 2x40ml brine, dried over MgSO
4 and the solvent evaporated under reduced pressure to give an amber oil, which was purified by flash chromatography on 30g silica gel using a gradient of EtOAc:MeOH to give an oil, which was crystallized from boiling hexane to give N,N-Dimethyltryptamine as colorless waxy crystals, weighing 0.9g (69%).