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Using this preamble:

documentclass[pdftex,11pt,a4paper,openany]{memoir}

usepackage[brazil]{babel}

usepackage[utf8]{inputenc}

usepackage[T1]{fontenc}

usepackage{indentfirst}

usepackage{latexsym}

usepackage{amsmath,amssymb,amsfonts,wasysym}

usepackage[pdftex]{graphicx}

usepackage{wrapfig}

usepackage[left=3cm,right=2cm,top=3cm,bottom=3cm]{geometry}

usepackage{color}

usepackage[dvipsnames]{xcolor}

definecolor{darkgray}{gray}{0.3}

setcounter{secnumdepth}{3}

setcounter{tocdepth}{3}

usepackage[pdftex, colorlinks=true, linkcolor=darkgray, citecolor=blue, urlcolor=blue]{hyperref}

TexMaker gives this error message:

! LaTeX Error: Command newfloat already defined.

Or name end... illegal, see p.192 of the manual.

Where is the problem?

There's no error with the preamble you provided. If you are trying to load the package float afterwards, don't: memoir has its built-in mechanism for defining new floating environments.

See p. 169 in the documentation.

Just for the record, I had the same problem, using memoir and floatrow together. Removing pdflatex from the preamble did not solve it.

I compiled the document successfully by adding the following line before loading floatrow:

letnewfloatundefined

usepackage{floatrow}

Thanks to Joseph Wright.